Background of the Invention
Field of the Invention
[0001] The present invention is in the field of fiber optic acoustic sensor arrays wherein
light is propagated in the arrays and the effects of acoustic signals on the light
returning from the arrays are analyzed to determine the characteristics of the acoustic
signals.
Description of the Related Art
[0002] Fiber optic based acoustic sensors are promising alternatives to conventional electronic
sensors. Included among their advantages are a high sensitivity, large dynamic range,
light weight, and compact size. The ability to easily multiplex a large number of
fiber optic sensors onto common busses also makes fiber optic sensors attractive for
large-scale arrays. The recent successful incorporation of multiple small-gain erbium
doped fiber amplifiers (EDFAs) into a fiber optic sensor array to increase the number
of sensors that can be supported by a single fiber pair has made large-scale fiber
optic sensor arrays even more competitive.
[0003] For acoustic detection, the fiber optic sensor of choice has been the Mach-Zehnder
interferometric sensor. In any interferometric sensor, phase modulation is mapped
into an intensity modulation through a raised cosine function. Because of this nonlinear
transfer function, a sinusoidal phase modulation will generate higher order harmonics.
An interferometer biased at quadrature (interfering beams π/2 out of phase) has a
maximized response at the first order harmonic and a minimized response at the second
order harmonic. For this reason, quadrature is the preferred bias point. As the bias
point drifts away from quadrature (for example, due to external temperature changes),
the response at the first order harmonic decreases and the response at the second
order harmonic increases. When the interferometer is biased at 0 or π out of phase,
the first order harmonic disappears completely. This decreased response at the first
order harmonic (resulting from the bias points away from quadrature) is referred to
as signal fading.
[0004] Because Mach-Zehnder interferometric sensors have an unstable bias point, they are
especially susceptible to the signal fading problem just mentioned. In order to overcome
signal fading, a demodulation of the returned signal is required. The typical demodulation
technique is the Phase-Generated Carrier (PGC) scheme, which requires a path-mismatched
Mach-Zehnder interferometric sensor. (See, for example, Anthony Dandridge, et al.,
Multiplexing of Interferometric Sensors Using Phase Carrier Techniques, Journal of Lightwave Technology, Vol. LT-5, No. 7, July 1987, pp. 947-952.) This path imbalance also causes the conversion
of laser phase noise to intensity noise, which limits the performance of the Mach-Zehnder
interferometric sensor arrays at low frequencies and places stringent requirements
on the linewidth of the source. This narrow linewidth requirement has slowed the development
of amplified Mach-Zehnder interferometric sensor arrays at 1.55 µm.
[0005] The Sagnac interferometer has found widespread use in the fiber optic gyroscopes.
(See, for example, B. Culshaw, et al.,
Fibre optic gyroscopes, Journal of Physics E (Scientific tnstruments). Vol.16, No. 1,1983, pp. 5-15.) It has been proposed that the Sagnac interferometer
could be used to detect acoustic waves. (See, for example, E. Udd,
Fiber-optic acoustic sensor based on the Sagnac interferometer, Proceedings of the SPIE-The International Society for Optical Engineering, Vol. 425, 1983, pp. 90-91; Kjell Krakenes, et al.,
Sagnac interferometer for underwater sound detection: noise properties, OPTICS LETTERS. Vol. 14, No. 20, October 15, 1989, pp. 1152-1145; and Sverre Knudsen, et al., An
Ultrasonic Fiber-Optic Hydrophone Incorporating a Push-Pull Transducer in a Sagnac
Interferometer, JOURNAL OF LIGHTWAVE TECHNOLOGY. Vol. 12, No. 9, September 1994, pp. 1696-1700.) Because of its common-path design,
the Sagnac interferometer is reciprocal and therefore has a stable bias point, which
eliminates signal fading and prevents the conversion of source phase noise into intensity
noise. Therefore, the Sagnac interferometer is immune to the phase noise which limits
the Mach-Zehnder interferometric sensors at low frequencies.
[0006] International application WO99/52323 describes a folded Sagnac fiber optic acoustic
sensor array 700 that operates in a manner similar to a Sagnac interferometer but
uses a common delay path to reduce distributed pickup in downlead fibres which may
be used to detect acoustic waves in water. By basing the fold Sagnac sensor array
on operating principles similar to the Sagnac interferometer rather than basing the
array on a Mach-Zehnder interferometer, the sensor array has a stable bias point,
has reduced phase noise, and allows a broadband signal source to be used rather than
requiring a more expensive narrowline laser.
Summary of the Invention
[0007] According to one aspect of the present invention there is provided an acoustic sensor
as recited in Claim 1. According to a second aspect of the present invention there
is provided a method of detecting acoustic signals as recited in Claim 13.
[0008] One embodiment of the present invention is an acoustic sensor that comprises a source
of light pulses, a first coupler, a polarization dependent second coupler, an optical
delay path and at least one detector. The first coupler couples the light pulses to
a first optical path having a first optical length and to an array of sensors. The
array of sensors comprises at least a first sensor. The first sensor is in a second
optical path having a second optical length different from the first optical length.
The polarization dependent second coupler couples light pulses received from the first
optical path in a first polarization to the optical delay path and couples light pulses
received from the array in a second polarization to the optical delay path. The light
pulses coupled to the optical delay path in the first polarization return from the
optical delay path to the second coupler in the second polarization. The light pulses
coupled to the optical delay path in the second polarization return from the optical
delay path to the second coupler in the first polarization. The second coupler couples
the light pulses returning to the second coupler from the optical delay path in the
first polarization to the first optical path to propagate therein to the first coupler.
The second coupler couples light pulses returning to the second coupler from the optical
delay path in the second polarization to the array to propagate therein to the first
coupler. The first coupler combines the light pulses from the first optical path and
the light pulses from the array to cause right pulses traveling equal distances through
the first optical path and the array to interfere and to generate a detectable output
signal. The detectable output signal varies in response to acoustic energy impinging
on the first sensor. The detector detects the detectable output signals to generate
a detector output signal responsive to variations in the detectable output signal
from the first coupler Preferably, the array includes a second sensor. The second
sensor is in a third optical path having a third optical length different from the
first optical length and the second optical length. Also preferably, the polarization
dependent second coupler comprises a polarization beam splitter. In preferred embodiments,
the optical delay path comprises a length of optical waveguide and a polarization
rotating reflector. The reflector causes light incident on the reflector in the first
polarization to be reflected as light in the second polarization, and causes light
incident on the reflector in the second polarization to be reflected as fight in the
first polarization. the reflector advantageously comprises a Faraday rotating mirror.
The first optical path includes a non-reciprocal phase shifter which causes light
propagating through the first optical path in a first direction and light propagating
through the first optical path in a second direction to experience a relative phase
shift such that light combined in the first coupler has a phase bias. Preferably,
In such embodiments, a third optical path is positioned in parallel with the first
optical path. One of the first optical path and the third optical path includes an
optical delay to cause the first optical path to have an optical path length different
from an optical path length of the third optical path, such that light propagating
through the first optical path has a propagation time different from a propagation
time of light propagating through the second optical path to thereby time multiplex
the light pulses. Preferably, the non-reciprocal phase shifter comprises a first Faraday
rotator, a quarter-wave plate and a second Faraday rotator, the first Faraday rotator.
The quarter-wave plate and the second Faraday rotator are positioned such that light
propagating in the first direction passes through the first Faraday rotator, then
through the quarter-wave plate, and then through the second Faraday rotator, and such
that light propagating in the second direction passes through the second Faraday rotator,
then through the quarter-wave plate, and then through the first Faraday rotator. Alternatively,
the non-reciprocal phase shifter comprises a first quarter-wave-plate, a Faraday rotator,
and a second quarter-wave plate. The first quarter-wave plate, the Faraday rotator,
and the second quarter-wave plate are positioned such that light propagating in the
first direction passes through the first quarter-wave plate, then through the Faraday
rotator, and then through the second quarter-wave plate, and such that light propagating
in the second direction passes through the second quarter-wave plate, then through
the Faraday rotator, and then through the first quarter-wave plate.
[0009] Another embodiment of the present invention is a method of detecting acoustic signals.
The method comprises generating an input light signal and coupling the input light
signal to at least first and second propagation paths to propagate in respective first
directions therein. The first and second propagation paths have respective first and
second optical lengths. The first and second propagation paths output respective first
and second output light portions. The first and second output light portions are output
from the first and second propagation paths at differing times in accordance with
differences in the first and second optical path lengths. The second output light
portion is modulated by an acoustic signal impinging on the second propagation path.
The first light portion is coupled to a delay path in a first polarization, and the
second light portion is coupled to the delay path in a second polarization. The delay
path outputs a first delayed light portion corresponding to the first output light
portion. The first delayed light portion has the second polarization. The delay path
outputs a second delayed light portion corresponding to the second output light portion.
The second delayed light portion has the first polarization. The first and second
delayed light portions are coupled to the first and second propagation paths to propagate
therein in respective second directions opposite the respective first directions.
The first propagation path outputs a first set of return light portions. The first
set of return light portions comprise a respective return light portion for each of
the first and second delayed light portions. The second propagation path outputs a
second set of return light portions. The second set of return light portions comprise
a respective return light portion for each of the first and second delayed light portions.
The first and second sets of return light portions are coupled to at least one detector.
The return light portions in the first and second sets of return light portions result
from output light portions and delayed light portions which travel identical optical
path lengths and interfere to generate detectable output signals. The method selectively
detects the detectable output signals to detect only output signals resulting from
interference of light portions which propagated in the first propagation path in either
the first direction or the second direction. The detectable output signals vary in
response to the acoustic signal impinging on the second propagation path.
[0010] Light propagating through a non-reciprocal phase shifter included in said first propagation
path in said first direction and light propagating through said non-reciprocal phase
shifter in said second direction experience a relative phase shift. reflector. The
length of optical fiber is selected to provide an optical delay time. The light propagates
through the optical fiber from the second coupler to the reflector. The reflector
reflects light into the optical fiber to propagate through the optical fiber to the
second coupler. The reflector further rotates light incident in the first polarization
to the second polarization and rotates light incident in the second polarization to
the first polarization. Preferably, the reflector comprises a Faraday rotating mirror.
Also preferably, the polarization dependent second coupler comprises a polarization
beam splitter positioned so that the delay path receives the light from a port of
the polarization beam splitter and returns light to the port of the polarization beam
splitter.
Brief Description of the Drawings
[0011] The present invention will be described below in connection with the accompanying
drawing figures in which:
Figure 1 illustrates an exemplary Sagnac interferometer having a single sensing loop;
Figure 2 illustrates a Sagnac sensor array in accordance with the present invention
wherein each rung of a sensor array forms an additional Sagnac interferometer,
Figure 3 illustrates a Sagnac sensor array which includes erbium-doped fiber amplifiers
to regenerate signal power lost to coupling and dissipative losses;
Figure 4 illustrates a graph of the frequency response of a Sagnac interferometer
in accordance with present invention compared with the three dominant ocean floor
noises;
Figure 5 illustrates graphs of the maximum and minimum acoustic signal detectable
by a Mach-Zehnder interferometer and detectable by a Sagnac interferometer in accordance
with the present invention, showing the relatively constant dynamic range of a Sagnac
interferometer over a wide range of frequencies;
Figure 6 illustrates graphs of the minimum detectable acoustic signal versus frequency
for three Sagnac interferometer configurations having different lengths of fiber in
the hydrophone and the delay loop;
Figure 7 illustrates a Sagnac interferometer in accordance with the present invention
which includes an additional delay loop to increase the dynamic range of the interferometer,
Figure 8 illustrates a graph of the dynamic range provided by the interferometer of
Figure 7;
Figure 9A illustrates the positioning of the delay loop of the interferometer in the
dry end of a sensor array system;
Figure 9B illustrates the positioning of the delay loop of the interferometer in the
wet end of a sensor array system;
Figure 10 illustrates the Sagnac interferometer of Figure 9B with annotations showing
the lengths used in calculations of the effects of phase modulation;
Figure 11 illustrates a technique for winding the delay loop so as to reduce the effects
of the acoustic wave upon the delay loop;
Figure 12 illustrates a Sagnac interferometer in accordance with the present invention
which includes empty rungs which detect distributed pick-up noise which can be subtracted
from the signals generated by the sensors;
Figure 13 illustrates a Sagnac interferometer in accordance with the present invention
which includes a depolarizer to reduce the effects of polarization induced fading;
Figure 14 illustrates a Sagnac interferometer which utilizes frequency divisional
multiplexing;
Figure 15 illustrates a graph which shows the generation of the beat signals between
the delayed modulation signal and the returning sensor signals in the interferometer
of Figure 14;
Figure 16 illustrates a Sagnac interferometer which utilizes code division multiplexing;
Figure 17 illustrates the architecture of a folded Sagnac acoustic fiber sensor array;
Figure 18 illustrates a graph of the number of returned pulses per time interval,
showing the separation in time of signal pulses and noise pulses;
Figure 19 illustrates a folded Sagnac acoustic fiber sensor array having a second
delay loop to provide extended dynamic range;
Figure 20 illustrates a folded Sagnac acoustic fiber sensor array having a phase modulator
and nulling circuitry in place of the reflector in Figure 17;
Figure 21 illustrates a further alternative embodiment of Figure 19 in which the two
delay loops are connected to different ports of the coupler,
Figure 22 illustrates an alternative embodiment of a fiber optic acoustic sensor array
system using a Faraday rotating mirror;
Figures 23A, 23B and 23C illustrate further alternative embodiments of a fiber optic
acoustic sensor array which utilize an unpolarized light source in combination with
a depolarizer, a polarization beam splitter and a Faraday rotating mirror;
Figure 24 illustrates an alternative embodiment of a folded fiber optic acoustic sensor
array which utilizes an unpolarized light source in combination with an optical circulator,
a 2×2 coupler, and a non-reciprocal phase shifter;
Figure 25 illustrates an alternative embodiment of a folded fiber optic acoustic sensor
array similar to Figure 24 in which the depolarizer is located in the second array
input/output fiber;
Figure 26 illustrates a first preferred embodiment of the non-reciprocal π/2 phase
shifter in Figures 24 and 25, which illustrates the effect on the polarization of
the light propagating in a first direction through the phase shifter,
Figure 27 illustrates the effect on the polarization of the light propagating in a
second (opposite) direction through the phase shifter of Figure 26;
Figure 28 illustrates an alternative preferred embodiment of the non-reciprocal π/2
phase shifter in Figures 24 and 25, which illustrates the effect on the polarization
of the light propagating in a first direction through the phase shifter;
Figure 29 illustrates the effect on the polarization of the light propagating in a
second (opposite) direction through the phase shifter of Figure 28;
Figure 30 illustrates a further alternative embodiment of a folded fiber optic acoustic
sensor array, which utilizes polarization-based biasing for multiple detectors, wherein
each detector has a bias point which can be set independently of the bias points of
the other detectors;
Figure 31 illustrates an alternative embodiment of a folded fiber optic acoustic sensor
array similar to Figure 30 in which the depolarizer is located in the second array
input/output fiber;
Figure 32 illustrates an alternative embodiment of a folded fiber optic acoustic sensor
array similar to Figure 30 in which an optical circulator replaces the 2×2 coupler,
Figure 33 illustrates an alternative embodiment of a folded fiber optic acoustic sensor
array similar to Figure 32 in which the depolarizer is located in the second array
input/output fiber;
Figure 34 illustrates a further alternative embodiment of a folded Sagnac sensor array,
which includes a combined input/output subsystem;
Figure 35 illustrates an alternative embodiment of a folded fiber optic acoustic sensor
array similar to Figure 34 in which the depolarizer is located in the second array
input/output fiber, and
Figure 36 illustrates a further alternative embodiment of a folded fiber optic acoustic
sensor array similar to Figures 34 and 35 in which the detectors are coupled to the
input/output subsystem by optical fibers to permit the detectors to be located remotely.
Detailed Description of the Preferred Embodiments
[0012] The present invention is described below in connection with an array of acoustic
sensors (e.g., hydrophones) in a Sagnac loop. Before describing the preferred embodiments,
a brief review of the operation of a single loop Sagnac acoustic sensor is provided.
Single Loop Sagnac Acoustic Sensor
[0013] A simple Sagnac-based acoustic sensor 100 is shown in Figure 1. The Sagnac loop is
divided into two portions, a delay loop 102 and a hydrophone 104. The delay loop 102
is simply a large length of fiber, typically greater than 1 km. The hydrophone 104
is a portion of fiber in which an acoustic wave is transformed into a phase modulation
of an optical signal propagating through the fiber. A high responsivity to acoustic
waves is typically accomplished by selecting optimized coatings for the section of
fiber in the hydrophone 104, and wrapping the fiber around a mandrel of suitable composition.
(See, for example, J.A. Bucaro, et al.,
Optical fibre sensor coatings, Optical Fiber Sensors, Proceedings of the NATO Advanced Study Institute, 1986, pp. 321-338.) The length of fiber wrapped around the hydrophone 104 is typically
10 meters to 100 meters. Light from a source 110, such as, for example, a superfluorescent
fiber source (SFS), is split into clockwise (CW) and counter-clockwise (CCW) beams
by a 3×3 coupler 112. The operation of the 3×3 coupler 112 is well-known and is described,
for example, in Sang K. Sheem,
Fiber-optic gyroscope with [3×
3] directional coupler, Applied Physics Letters, Vol. 37, No. 10, 15 November 1980, pp. 869-871..
[0014] Although described herein as using a 3×3 coupler 112, other couplers (e.g., a 2×2
coupler, a 4x4 coupler, etc.) can be used with alternative embodiments of the present
invention. For example, to use a 2×2 coupler, both ports of one side are used to create
the Sagnac interferometer. One port of the other side is a detection port. The remaining
port is used to launch light into the array and can also be used as a detection port
if a coupler or circulator is employed (in a similar manner as is done with fiber
optic gyroscopes). In general, any n×m coupler can be employed by using two ports
of one side of the coupler to create the Sagnac interferometer and using the ports
on the other side of the coupler as detection ports, launching ports, or both.
[0015] After splitting, the CW beam travels through the delay loop 102 first and then through
the hydrophone 104, while the CCW beam travels through the hydrophone 104 first and
then through the delay loop 102. During a time delay T
delaybetween a time when the CW beam travels through the hydrophone 104 and a time when
the CCW beam travels through the hydrophone 104, the acoustic signal and likewise
the acoustically induced phase modulation in the hydrophone 104 changes. This change
in phase modulation is mapped into a phase difference between the counter-propagating
beams, which is converted into an intensity modulation when the beams recombine at
the 3×3 coupler 112. This intensity modulation is then detected by a first detector
120 and a second detector 122 or by only one of the two detectors.
[0016] More explicitly, if an acoustic signal induces a phase modulation Ø
hcos(Ωt) in the fiber of the hydrophone 104, the resulting phase modulation between
the interfering beams at the hydrophone 104, φ
int(t), is given by:
where T
delay is the travel time through the delay loop. Thus, φ
int(t) is a function of the hydrophone modulation φ
h and the product of the acoustic modulation frequency, Ω, with the loop delay, T
delay. This differs from a Mach-Zehnder interferometric sensor in which φ
int(t) is a function of only the hydrophone modulation φ
h. Maximum sensitivity is achieved in the Sagnac loop acoustic sensor when the product
of the acoustic frequency, Ω, and the time delay, T
delay, is an odd multiple of π (maximum value of the first sine term in Equation 1). The
acoustic frequency which makes this product π is called the proper frequency of the
loop, which is the lowest frequency at which maximum sensitivity is achieved. Most
underwater sensing applications are concerned with the detection of acoustic frequencies
below 10 kHz. For the proper loop frequency to be less than 10 kHz, a delay time of
at least 50 microseconds and therefore a delay loop length of at least 10 km is required.
Thus, the Sagnac acoustic sensor 100 requires a large amount of fiber for the detection
of low acoustic frequencies (<10 kHz).
[0017] The common-path design inherent to the Sagnac interferometer has many advantages
over a Mach-Zehnder interferometer in addition to the stable bias point and elimination
of phase noise already mentioned. A Sagnac interferometer allows the use of a short-coherence
length, broadband source, such as a superfluorescent fiber source (SFS), an example
of an amplified spontaneous emission (ASE) source. Such sources are inexpensive and
can readily provide high powers. It has been shown that the use of the 3x3 coupler
passively biases the Sagnac acoustic sensor near quadrature. (See, Sang K. Sheem,
Fiber-optic gyroscope with [3×
3] directional coupler, Applied Physics Letters, Vol. 37, No. 10, 15 November 1980, pp. 868-871; and H. Poisel, et al.,
Low-cost fibre-optic gyroscope, Electronics Letters, Vol. 26, No. 1, 4
th January 1990, pp. 69-70.) By subtracting the signals from the two detection ports
of the 3x3 coupler, the source excess noise, which is the limiting noise source of
SFS sources, can be subtracted while phase-modulation induced intensity variations
due to the hydrophone are added. This allows a Sagnac interferometer to approach near
shot-noise limited performance. (See, Kjell Krakenes, et al.,
Sagnac interferometer for underwater sound detection: noise properties, OPTICS LETTERS, Vol. 14, No. 20, October 15, 1989, pp. 1152-1145.)
[0018] Previous work on Sagnac-based acoustic sensors has been limited to a single sensor
configuration. Because of the inherent advantages of the Sagnac interferometer, Applicants
have determined that it is desirable to replace the Mach-Zehnder interferometric sensors
in a large-scale array with Sagnac based sensors. Each Sagnac sensor 100 discussed
above requires many kilometers of fiber, making the insertion of numerous such sensors
into a large-scale array impractical. Research into using recirculating delay loops
to reduce the fiber length requirement has produced sensors which use significantly
less fiber but suffer from high noise due to the incorporation of EDFAs within the
recirculating loop. (See, for example, J.T. Kringlebotn, et al.,
Sagnac Interferometer Including A Recirculating Ring With An Erbium-doped Fibre Amplifier, OFS '92 Conference Proceedings, pp. 6-9.) A novel approach for decreasing the fiber required is described below.
Novel Sensor Array Based on the Sagnac Interferometer
[0019] As set forth below, Applicants have discovered a novel system which reduces the amount
of fiber needed for a Sagnac-based large scale array by multiplexing multiple sensors
onto the same delay loop, producing a practical Sagnac sensor array (SSA). As illustrated
in Figure 2, a Sagnac sensor array 200 in accordance with the present invention includes
an array 210 of hydrophones 212(i) in a ladder configuration which are attached to
a single delay loop 214. For example, Figure 2 shows a Sagnac sensor array 210 having
N hydrophones 212(1), 212(2) ... 212(N) in respective rungs 216(1), 216(2) ... 216(N).
Each rung 216(i) in the Sagnac sensor array 210 comprises a single fiber wrapped around
a respective hydrophone 212(i). Every path from a 3×3 coupler 220 through the delay
loop 214 and array 210 and back to the coupler 220 comprises a separate Sagnac interferometer.
Therefore, for an array of N sensors 212, there are N separate Sagnac interferometers,
each of which behaves like the single loop Sagnac sensor 100 shown in Figure 1. Each
Sagnac interferometer measures the acoustic signal at a separate point in space, i.e.,
the location of the hydrophone 212(i). For example, the Sagnac interferometer comprising
the delay loop 214 and the rung 216(1) measures the acoustic signal at hydrophone
212(1). In addition, each Sagnac interferometer also picks up acoustic signals (e.g.,
noise) elsewhere in the loop, which noise is advantageously reduced, as will be discussed
below.
[0020] The Sagnac sensor array 200 is easiest understood in a time-division multiplexed
(TDM) configuration (non-TDM schemes are discussed later). A source 222 (which may
advantageously comprise a conventional pulsed source or may comprise a cw source with
an external modulator) generates a light pulse which enters the Sagnac loop via a
third port of the coupler 220 and propagates in both the CW and CCW directions as
indicated in Figure 2. Upon reaching the array 210, the CCW pulse is split into a
train of N separate pulses. At this point, the CW input pulse has not yet reached
the array 210 and is still a single pulse. When the CW pulse reaches the array 210,
it also is split into a train of N pulses. Each pulse in the CW train returns to the
3x3 coupler 220 after traveling through a respective rung 216(i) and interferes with
the pulse in the CCW train which has traveled the same rung 216(i) in the opposite
direction. Thus, N pulses are detected by a first detector 230 and a second detector
232, and each pulse comprises the CW and CCW pulses of one of the N Sagnac loops (i.e.,
the two pulses which have traveled in opposite directions through the same respective
rung 216(i)). Because the pulses which travel through different combinations of rungs
do not travel identical optical paths, such pulses are not coincident in time at the
coupler 220, and thus do not interfere with each other at the coupler 220. The pulse
widths should be smaller than the differential delay between adjacent sensors so that
the pulses from adjacent sensors do not overlap.
[0021] As illustrated in Figure 3, small-gain erbium doped fiber amplifiers (EDFAs) 240
are advantageously added to the array portion 210 just as EDFAs have been added to
Mach-Zehnder interferometric sensor arrays. (See, for example, Craig W. Hodgson, et
al.,
Optimization of Large-Scale Fiber Sensor Arrays Incorporating Multiple Optical Amplifiers-Part
I: Signal-to-Noise Ratio, JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol.16, No. 2, February 1998, pp. 218-223; Craig W. Hodgson, et al.,
Optimization of Large-Scale Fiber Sensor Arrays Incorporating Multiple Optical Ampliriers-Part
II: Pump Power, JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol.16, No. 2, February 1998, pp. 224-231; Jefferson L. Wagener; et al.,
Novel Fiber Sensor Arrays Using Erbium-Doped Fiber Amplifiers, JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 15, No. 9, September 1997, pp. 1681-1688; and C.W. Hodgson, et al.,
Large-scale interferometric fiber sensor arrays with multiple optical amplifiers, OPTICS LETTERS, Vol. 22, No. 21, November 21, 1997, pp. 1651-1653.) The EDFAs 240 increase the number
of sensors which can be supported by a single array 210 by regenerating the signal
power which is lost to coupling and dissipative losses. The EDFAs are advantageously
pumped by one or more pump laser sources 242 via a splitting coupler 244 and via a
first wavelength division multiplexing (WDM) coupler 246 and a second WDM coupler
248.
[0022] Because it uses the Sagnac architecture, the Sagnac sensor array 200 has all of the
advantages of the single loop Sagnac based sensor 100 discussed above. The common-path
design eliminates the conversion of source phase noise into intensity noise at the
interfering coupler 220. The source 222 can be a fiber ASE (amplified spontaneous
emission) source (i.e., the SFS discussed above), which provides high powers inexpensively
at 1.55 µm. Passive biasing near quadrature is achievable for all sensors by using
the 3×3 coupler 220. Also, the 3×3 coupler 220 provides a convenient means to detect
two interferometric outputs at the detectors 230, 232, and to use the outputs of the
two detectors to subtract source excess noise. (See, for example, K. Krakenes, et.
al.,
Sagnac interferometer for underwater sound detection: noise properties, OPTICS LETTERS, Vol. 14, 1989, pp. 1152-1154, which shows the use of two detectors in combination
with a single Sagnac interferometer.)
[0023] The properties of this novel Sagnac sensor array 200 will be discussed more specifically
below followed by a more detailed discussion of the frequency response and dynamic
range which result from the use of a Sagnac interferometer. Thereafter, a calculation
of the magnitude of the distributed pick-up from the non-hydrophone fiber loop segments
will be described, along with a technique for reducing this pick-up magnitude. Polarization
will also be addressed below. New sources of noise which are introduced by the Sagnac
design are then discussed. Finally, multiplexing schemes other than TDM for the Sagnac
sensor array are presented.
[0024] Although the present invention is described above with respect to a single sensor
in each rung 216(i) of the array-210, it should be understood that each rung 216(i)
may advantageously comprise a subarray having multiple sensors, such as are described,
for example, in allowed U.S. Patent Application No. 08/814,548, filed on March 11,
1997, which is incorporated by reference herein. (See, also, C.W. Hodgson, et al.,
Large-scale interferometric fiber sensor arrays with multiple optical amplifiers, Optics Letters, Vol. 22, 1997, pp. 1651-1653; J.L. Wagener, et al.,
Novel fiber sensor arrays using erbium-doped fiber amplifiers, Journal of Lightwave Technology, Vol.15, 1997, pp. 1681-1688; C.W. Hodgson, et al.,
Optimization of
large-scale fiber sensor arrays incorporating multiple optical amplifiers, Part I:
signal-to-noise ratio, Journal of Lightwave Technology, Vol. 16, 1998, pp. 218-223; and C.W. Hodgson, et al.,
Optimization of large-scale fiber sensor arrays incorporating multiple optical amplifiers,
Part II. pump power, Journal of Lightwave Technology, Vol.16, 1998, pp. 224-231.)
Frequency Response
[0025] As set forth above, the Sagnac sensor has a frequency dependent response given by
Equation 1. At frequencies well below the proper frequency of the loop, defined as
1/(2·T
delay), the minimum detectable acoustic signal scales with the inverse of acoustic frequency.
This decreased acoustic sensitivity at low frequencies has been a major concern for
the Sagnac acoustic sensor. However, it has been pointed out that this decreased sensitivity
at low frequencies is fortunately matched by an increasing ocean noise floor (See,
for example, Sverre Knudsen,
Ambient and Optical Noise in Fiber-Opfic Interferometric Acoustic Sensors, Fiber-Optic Sensors Based on the Michelson and Sagnac Interferometers: Responsivity
and Noise Properties, Thesis, Chapter 3, Norwegian University of Science and Technology, 1996, pp. 37-40.)
Ideally, it would be desirable if the minimum detectable acoustic signal of an array
at a given frequency were to be a constant amount below the ocean noise floor at that
frequency. Thus, the minimum detectable acoustic signal would also increase at lower
frequencies to match the increasing ocean noise floor. The frequency response of the
Sagnac sensor array 200 of the present invention in fact does provide a good match
between the ocean noise floor and acoustic sensitivity. This is illustrated in Figure
4, where the minimum detectable acoustic signal for a Sagnac sensor array is plotted
as a curve 250 assuming an optical noise floor of
a hydrophone phase responsivity of 3.2 × 10
-7 rad/µPa and a delay loop length of 20 km. (The vertical axis is in dB relative to
a baseline of
Also plotted in Figure 4 are the ocean noise floors for the three dominant ocean
noise sources at these frequencies and a resulting sum of the noise from the three
sources. A curve 252 represents the noise from ocean turbulence, earthquakes, volcanic
eruptions, and the like. A curve 253 represents light shipping noise. A curve 254
represents DSS0 (distant shipping and storms) noise. A curve 256 represents the sum
of the noise floors from the three dominant sources (i.e., the sum of the curves 252,
253 and 254). (See, for example, Robert J. Urick,
The noise background of the sea: ambient noise level, Principles of Underwater Sound, 3rd Ed., Chapter 7, McGraw-Hill, 1983, pp. 202-236.) The minimum detectable acoustic
signal of the Sagnac sensor array 200 increases in such a way as to provide a nearly
constant amount of detectable signal below the ocean noise floor at all frequencies
below 10 kHz. Thus, the frequency-dependent response of the Sagnac sensor array 200
does not prohibit low-frequency acoustic detection. The Mach-Zehnder array shows the
same trend as the Sagnac sensor array, namely a decreasing sensitivity towards lower
frequencies, but in the Mach-Zehnder array, the decreasing sensitivity is smaller
than in the Sagnac-based sensor. Although both the Mach-Zehnder interferometer and
Sagnac sensor array 200 have similar frequency-dependent responses, the source of
their frequency responses is fundamentally different. The increasing minimum detectable
signal in the Mach-Zehnder interferometer sensor array is due to an increasing optical
noise floor. The cause of this increasing optical noise floor is the phase noise introduced
by the path-imbalanced Mach-Zehnder interferometer. Thus, although the noise floor
is
at 10 kHz, it increases towards lower frequencies. In the Sagnac sensor array 200,
the increasing minimum detectable acoustic signal is due to the sin(ΩT
delay/2) term in Equation 1, and not to an increasing optical noise floor. The optical
noise floor remains a constant
over the entire frequency range.
[0026] The significance of this difference can be seen by examining the dynamic range of
the Mach-Zehnder interferometric sensor array and Sagnac sensor array 200, illustrated
in Figure 5. The dynamic range of a sensor is limited by the minimum and maximum detectable
phase shifts. For interferometric sensors, the maximum detectable phase shift is limited
by the nonlinear response of the interferometer and the minimum detectable phase shift
by the optical noise floor. Both the Mach-Zehnder interferometric sensor array and
the Sagnac sensor array have maximum detectable phase shifts which are constant over
the acoustic frequency range. However, the Sagnac sensor array 200 also has a flat
minimum detectable phase shift because it has a flat optical noise floor, while the
Mach-Zehnder interferometric sensor array suffers an increasing minimum detectable
phase shift due to an increasing optical noise floor caused by the phase noise introduced
by the path imbalanced interferometer. The Sagnac sensor array 200 thus has a constant
dynamic range at all acoustic frequencies, while the Mach-Zehnder interferometric
sensor array has a decreased dynamic range at low acoustic frequencies. This is illustrated
in Figure 5, wherein the minimum and maximum detectable acoustic signals (in dB arbitrary
units) are plotted for the Sagnac sensor array 200 and a Mach-Zehnder interferometric
sensor array. As shown in Figure 5, both arrays have an approximately 100 dB dynamic
range above 1 kHz, where phase noise does not limit the Mach-Zehnder interferometric
sensor array. At 10 Hz, phase noise dominates the Mach-Zehnder interferometric sensor
array, and its dynamic range is reduced to approximately 74 dB. Meanwhile, the dynamic
range of the Sagnac sensor array 200 remains at approximately 100 dB.
[0027] It is interesting to examine the frequency response of the Sagnac sensor array 200
at frequencies well below the loop proper frequency as a function of the delay loop
length and hydrophone responsivity. At these frequencies, the sin(ΩT
delay/2) factor in Equation 1 can be approximated as ΩT
delay/2, showing that the responsivity of the Sagnac sensor array 200 is proportional to
the product of φ
h and T
delay. φ
h itself is proportional to the amount of fiber in each hydrophone 212(i), and T
delay is proportional to the amount of fiber in the delay loop 214. Thus, the responsivity
at frequencies well below the loop proper frequency is proportional to the product
of the hydrophone fiber length and delay fiber length. Figure 6 plots the minimum
detectable acoustic signal for several Sagnac sensor array configurations in which
the product of the length of the fiber in each hydrophone 212(i) and the length of
the fiber in the delay loop 214 is constant, but the relative distribution of fiber
between the delay loop 214 and each hydrophone 212(i) changes. For example, a curve
260 represents the frequency response of a Sagnac sensor array 200 having 45 km of
fiber in its delay loop 214 and 100 meters of fiber in each hydrophone 212(i); a curve
262 represents the frequency response of a Sagnac sensor array 200 having 30 km of
fiber in its delay loop 214 and 150 meters of fiber in each hydrophone 212(i); and
a curve 264 represents the frequency response of a Sagnac sensor array 200 having
15 km of fiber in its delay loop 214 and 300 meters of fiber in each hydrophone 212(i).
As illustrated, each Sagnac sensor array 200 has the same sensitivity at low frequencies,
but approaches a maximum sensitivity at different frequencies given by their respective
loop proper frequencies. Thus, for a given minimum detectable acoustic signal at low
frequencies, there is still some freedom in choosing the fiber lengths of the delay
loop 214 and the hydrophones 212(i). This freedom may be used to help the Sagnac sensor
array 200 satisfy other criteria, such as minimizing the total amount of fiber required
or minimizing the delay loop length.
Increasing the Dynamic Range of the Sagnac sensor array
[0028] As discussed above, the Sagnac sensor array 200 has a larger dynamic range at low
acoustic frequencies than the Mach-Zehnder interferometric sensor array because it
is immune to phase noise. Ideally, an array 200 provides enough dynamic range to detect
the strongest and weakest acoustic signal which are likely to be encountered. This
requirement often translates into a required dynamic range of approximately 150 dB.
In order to achieve such a large dynamic range in a Mach-Zehnder interferometric sensor
array, two separate sensors with different phase responsivities are required, with
each detecting a fraction of the total 150 dB dynamic range. The obvious disadvantage
to this scheme is that it requires two sensor arrays (i.e., twice as many hydrophones,
rungs, sources and detectors). Effectively, an array which can support N hydrophones
can detect the acoustic signal at only N/2 points.
[0029] In the Sagnac sensor array 200, it is possible to achieve a large dynamic range without
using additional hydrophones 212. Because the phase responsivity in the Sagnac sensor
array is a function of the hydrophone responsivity and delay loop length, as shown
in Equation 1, the phase responsivity of the entire array of hydrophones can be changed
by modifying the delay loop length. By simultaneously using two separate delay loops
214(1) and 214(2) of length L
1 and L
2, respectively, as shown in a modified sensor array 266 in Figure 7, the detection
range of the array 266 can be dramatically increased. The array 266 now has 2N separate
Sagnac loops. Each hydrophone 212(i) returns a separate signal for each of the two
delay loop paths, and the length of each delay loop 214(1), 214(2) determines the
acoustic detection range of that signal. The total acoustic detection range of each
hydrophone 212(i) is the union of the detection ranges of each of the two Sagnac loop
sensors which enclose the hydrophone 212(i). The lengths of L
1 and L
2 set the acoustic detection range. The length L
1+L
2 is chosen to allow the array 266 to detect the smallest acoustic signal of interest.
The length L
1 of the delay loop 214(1) is then chosen to place the detection range of the signals
which travel only this shorter delay loop on top of the detection range of the signals
which travel both delay loops 214(1), 214(2). In a TDM system, as a result of the
insertion of a second loop, the repetition frequency of the source pulses are halved
in order to allow time for 2N pulses to return, and the lengths of the delay loops
214(1), 214(2) are chosen such that there is no pulse overlap. Because the repetition
frequency is halved, the dynamic range of each individual signal decreases by 3 dB.
This decrease is more than offset by the increase in the total dynamic range achieved
by piggybacking the dynamic range of two separate signals. In Figure 7, the second
delay loop 214(2) is positioned such that all light passing through the second delay
loop 214(2) passes through the first delay loop 212(1). It should be understood that,
alternatively, the two delay loops 214(1), 214(2) can be optically in parallel such
that the light which passes through the second delay loop 214(2) does not pass through
the first delay loop 214(1). In such case, the fiber length of the second delay loop
214(2) would have to be the sum of the first length and the second length (i.e., L
1+L
2). But, since L
1 is considerably shorter than L
2, this adjustment is not essential. The embodiment of Figure 7 reduces the total fiber
requirements by adding the length of the first delay loop to the second delay loop.
[0030] Figure 8 illustrates the extended dynamic range made possible by using the two delay
loops 214(1), 214(2) in the array 266 in which the dynamic range of each signal is
100 dB and the ratio L1/L2 was set to be 5000. As shown, the array 266 is now able
to detect over the entire dynamic range of interest (approximately a 160-dB range)
without increasing the hydrophone count.
Distributed Sensing
[0031] In the Sagnac sensor array 266, any phase modulation in the interferometer can be
transferred into an intensity modulation at the interfering 3×3 coupler 220. This
distributed sensing over the entire Sagnac loop is disadvantageous for an acoustic
sensor array. In order to be practical, the acoustic sensor array should sample the
acoustic signal at a number of discrete points in space (i.e., at the hydrophones)
and return these signals independently. Mach-Zehnder interferometric sensor arrays
achieve this because the interferometer is confined within a small space and thus
only senses at that point. In order for the Sagnac sensor array 266 to be practical,
the distributed sensing of the Sagnac loop must be decreased.
[0032] The bulk of the fiber in the interferometer constitutes the delay loop 214, which
can be located in two positions. The first is with the source 222 and the detection
electronics (i.e., the detector 230 and the detector 232) in the dry end (i.e., out
of the water), as shown in Figure 9A. Here the delay loop 214 can be environmentally
shielded to minimize any external modulation. However, downlead fibers 270, 272 which
connect the wet end to the array portion 210 are part of the interferometer. The second
possibility is to locate the delay loop 214 in the wet end (i.e., in the water) with
the array 210, as shown in Figure 9B. As such, the delay loop 214 cannot be isolated
to the same extent as it could if it were located in the dry end, but the downlead
fibers 270, 272, 274 are outside of the interferometer and thus are non-sensing. The
relative magnitude of the downlead and delay loop distributed pick-up dictates which
configuration is best suited for a particular application, It should be noted that
if the delay loop 214 is located in the dry end (Figure 9A), the downlead fibers 270,
272 must remain stationary to prevent physical movements, such as bending and vibrations,
of these fibers, which can induce extremely large phase modulations. These are fiber
motion induced phase modulations as opposed to acoustically-induced phase modulations.
(Such physical movements are problems in towed arrays, but may not be significant
problems in stationary arrays.) Thus, if the delay loop 214 is located in the dry
end (Figure 9A), the entire wet end of the Sagnac sensor array 210 must be stationary.
However, with the delay loop 214 located in the wet end (Figure 9B), only the portion
to the right of the 3x3 coupler 220 in Figure 9B must remain stationary since the
downlead fibers 270, 272, 274 are not then part of the interferometer. When the delay
loop 214 is located in the wet end (Figure 9B), the delay loop fiber must be desensitized.
The delay loop 214 can be made stationary by wrapping the delay loop fibers around
a desensitized cylinder (not shown), thereby eliminating fiber motion and making acoustic
pick-up the dominant source of distributed pick-up signal. Because it is easier to
desensitize fiber to acoustically-induced phase modulation than it is to desensitize
fiber to movement-induced phase modulation, the configuration which locates the delay
loop 214 in the wet end (Figure 9B) is preferable for towed array applications and
will be described in more detail below.
Calculation of the Acoustic Pick-up Noise Induced in the Delay Loop
[0033] In this section, estimates are derived for the magnitude of the acoustically induced
distributed pick-up noise as compared to the acoustically induced hydrophone phase
modulation in the Sagnac sensor array 210 of Figure 9(b). The intensity modulation
due to the distributed phase modulations resulting from the pick-up of acoustic signals
in the delay loop and bus fiber (the fiber connecting each hydrophone to the delay
loop and the 3×3 coupler) can be considered a source of noise. For the following discussion,
consider one loop of the Sagnac sensor array as comprising only delay fiber of length
Ld, a bus fiber of length
Lb, a hydrophone fiber of length
Lh, and a total length L, as shown in Figure 10. Also assume that
Ld is much larger than
Lb and
Lh. The phase responsivity of fiber to acoustic signals results from a pressure dependent
propagation constant, β. In general, the pressure dependent component of the propagation
constant at a position / and time t can be written as:
where β
o is the zero-pressure propagation constant,
R(l) is the normalized phase responsivity of the fiber, and
P(l,t) is the pressure as a function of space and time. If a sinusoidal acoustic signal
of frequency Ω is assumed, Equation 2 can be rewritten as:
where
P0 is the steady-state pressure,
Pm is the amplitude of the pressure modulation (assumed to be independent of
l). and
θ(l) contains the spatial phase variation of the acoustic wave. In general, the induced
phase difference between interfering beams in a Sagnac loop due to acoustically induced
phase modulation from
I=
I1 to
I=I2 is given by the integral:
where ν is the speed of light in the fiber, and
L is the loop length. Substituting Equation 3 into Equation 4 yields:
Equation 5 can be used to determine the phase difference between interfering beams
due to acoustic modulation of the hydrophone, bus, and delay fibers.
[0034] For the hydrophone fiber, Equation 5 is integrated from
lt=ld+lb/2 to
l2=
ld+
lb/2+
lh. It is assumed that θ
(l) is constant over this range (i.e., that the acoustic wavelength is much larger than
the dimension of the hydrophone). It is also assumed that the normalized phase responsivity
of the fiber,
R(l), is constant and is equal to
Rb in this range. Equation 5 then gives a phase difference amplitude between interfering
beams due to hydrophone fiber modulation:
where it is assumed that Ω
Lh/2ν «1Note that Equation 2 agrees with the expression given in Equation 1.
[0035] For the bus fiber, Equation 5 is integrated first from
I1=Id to
I2=Id+Ib/2, and then from
I1=
L-Ib/2 to I
2=L to include both the upper and lower bus lines. Again, it is assumed that
R(I) is constant and equal to
Rb for all bus fiber, such that θ
(l) is constant in the integral of Equation 5. The phase difference amplitude between
interfering beams due to fiber modulation becomes:
where it is assumed that Ω
Lh/2ν «1It should be emphasized that the assumptions on the constancy of θ(
l) and the amplitude of Ω
Lh/2ν act to increase
thus giving a worst case scenario for the bus fiber.
[0036] For the delay fiber, Equation 5 is integrated from
l1=0 to
l2=ld, and, as before, it is assumed that θ(
l) is constant over this range (i.e., the delay loop coil is much smaller than the
acoustic wavelength), and that
R(l) is constant and equal to
Rd over the integral. Equation 5 then yields a phase difference amplitude between interfering
beams due to delay fiber modulation given by:
where it is assumed that
Ω(Lh+Lh)/
2v.
[0037] With Equations 6-8, the relative magnitude of these phase modulations amplitudes
can be computed. First, it is noted that a standard plastic coated fiber has a normalized
phase responsivity,
R, of -328 dB re 1/µPa, as described, for example, in JA Bucaro, et al.,
Optical fibre sensor coatings, Optical Fiber Sensors. Proceedings of the NATO Advanced Study Institute, 1986, pp. 321-338. On the other hand, as described, for example,
in C.C. Wang, et al.,
Very high responsivity fiber optic hydrophones for commercial applications, Proceedings of the SPIE-The International Society for Optical Engineering. VoL 2360, 1994, pp. 360-363, a fiber wrapped around current
hydrophones made from air-backed mandrels has a normalized phase sensitivity of -298
dB re 1/µPa, an increase of 30 dB over standard fiber. If we assume that the delay
loop and the bus fiber have the normalized phase responsivity of standard plastic
coated fiber, and that the hydrophone fiber is wrapped around an air-backed mandrel,
then the ratio of
Rh to
Rb or
Rd is approximately 30 dB. Therefore, under the simplifying assumption made to reach
Equations 6-8, it can be found that:
and
[0038] The ratio
Lb/
Lh is a function of the hydrophone position. For the first hydrophone,
Lb/
Lh≈0 making
and
extremely large. For the last hydrophone, typical values of 100 meters and 1 km for
Lh and
Lb, respectively, are used to arrive at
Thus, despite the fact that the hydrophone fiber constitutes a relatively small amount
of the overall Sagnac loop, the magnitude of the acoustically induced phase modulations
in the hydrophone fiber are greater than the acoustically induced phase modulations
in the delay loop fiber and in the bus fiber for even the furthest hydrophone. The
following section describes a means for dealing with this level of distributed pick-up
noise using empty rungs.
[0039] In order to evaluate the integral in Equation 5 for the delay loop fiber, it is assumed
that
R(I)=Rd for all / less than
Ld. It was this constancy of
R(l) which eliminated any contribution to the integral of Equation 5 from
I=(L-Ld) to
Ld (because the integrand became an odd function about
L/
2). However, coiling a long length of fiber will result in some dependence in
R(I) on / (possibly because the inner layer of fiber has a different
R than the outer layer). These variations in
R(I) increase the delay loop pick-up from
I=L-Ld to
Ld. In order to reduce this pick-up, it is first noted that
R(I) need only be an even function around
L/
2 to make the integrand of Equation 5 an odd function about
L/
2. R(l) can be forced to be more symmetric about
L/
2 by wrapping the delay loop in such a way as to position symmetric points of the fiber
loop next to each other as shown in Figure 11. Such a wrapping ensures that symmetric
points of the delay loop are positioned in proximity to each other so that any variations
in R(l) due to the position of the fiber on the coil are as symmetric about
L/
2 as possible, thereby making the delay loop pick-up as close to the expression of
Equation 8 as possible. Note that, because each Sagnac loop in the Sagnac sensor array
has a different
L/
2 point, only one loop can be wrapped exactly as shown in Figure 11, thereby introducing
a small degree of oddness in
R(I) to all but one of the Sagnac loops.
[0040] It should also be mentioned that in addition to enhancing the acoustic sensitivity
of fiber with a hydrophone, it is possible to desensitize fibers by applying a metallic
coating of a particular diameter. (See, for example, J.A. Bucaro,
Optical fibre sensor coatings, cited above.) Measured normalized phase responsivities as low as -366 dB re 1/µPa
have been reported. If such fibers are used in the delay or bus lines, the ratio of
Rh to
Rb or the ratio of
Rh to
Rd approaches 68 dB (instead of 30 dB with plastic coated delay and bus fibers), increasing
the hydrophone induced signal over the delay and bus induced signal by 38 dB.
Reducing the Distributed Pick-up Noise by Using Empty Rungs
[0041] In order to further eliminate distributed pick-up signal, the hydrophone-induced
acoustic modulation can be isolated from the distributed pick-up modulation by placing
empty rungs 300 that do not contain a hydrophone in the array 210, as shown in Figure
12. Each rung 216(i) which contains a hydrophone 212(i), called a sensing rung, is
proceeded by one of the empty rungs 300(i). The fact that the non-sensing fiber of
each loop which encloses an empty rung 300(i) is nearly identical to the non-sensing
fiber of the loop which encloses the corresponding sensing rung 212(i) means the empty
rung 300(i) and the corresponding sensing rung 212(i) will have nearly the same distributed
pick-up signal. By treating this empty rung 300(i) as another sensor in the array
210 and properly timing the pulses (in the TDM scheme) from the empty rungs 300(i)
and the sensing rungs 212(i) so that they do not overlap, the distributed pick-up
signal present on each sensing rung 212(i) can be measured. After detection, this
signal can be subtracted from the sensing rung signal, leaving only intensity variations
produced by phase modulations in the hydrophone fiber. Implementing such a scheme
requires 2N rungs for an N sensor array 210, thereby reducing the duty cycle of individual
signals by one half.
[0042] If desensitizing the bus portion of the array 210 is not required, a single empty
rung 300 can be placed in the array 210 to measure the distributed pick-up signal
associated with the delay loop 214, thereby requiring only N+1 rungs (N sensing rungs
212(i) and one empty rung 300) for N sensors. If one empty rung 300 does not adequately
measure the distributed pick-up signal for each sensing rung 212(i), more empty rungs
300 can be added at periodic intervals along the array, until the distributed pick-up
signal present on each sensing rung 212(i) can be adequately measured by the nearest
of these empty rungs 300. Using fewer empty rungs results in a higher duty cycle for
individual signals. Figure 12 depicts the extreme in which an empty rung was added
for every sensing rung.
Polarization
[0043] For maximum contrast in any interferometric sensor, the state of polarization (SOP)
of the interfering beams must be identical when they recombine. If they are orthogonal,
there is no interference and thus no amplitude-modulated signal. This is referred
to as polarization-induced signal fading. Because each sensor in the Sagnac sensor
array is a Sagnac loop, the research carried out so far on polarization-induced signal
fading in the Sagnac fiber gyroscope applies to the Sagnac sensor array as well. One
promising solution is to place a depolarizer within the Sagnac loop. (See, for example,
K. Böhm, et al.,
LOW-DRIFT FIBRE GYRO USING A SUPERLUMINESCENT DIODE,
ELECTRONICS LETTERS, Vol. 17, No. 10, 14th May 1981, pp. 352-353.) The depolarizer ensures that at least
half of the optical power is returning to the 3x3 coupler in the correct SOP at all
times. This general approach produces a constant visibility regardless of the loop
birefringence. (See, for example, William K. Burns, et al.,
Fiber-Optic Gyroscopes
with Depolarized Light, JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 10, No. 7, July 1992, pp. 992-999). The simplest configuration uses an unpolarized
source such as a fiber superfluorescence source and a depolarizer in the loop. As
illustrated in Figure 13, in the Sagnac sensor array 200, one depolarizer 310 is placed
at a point which is common to all the Sagnac loops. The depolarizer 310 ensures that
each sensor 212(i) has this constant visibility independent of birefringence as long
as the loop birefringence remains constant This represents a great simplification
in the handling of polarization-induced signal fading over those methods used in Mach-Zehnder
interferometric sensor arrays.
[0044] Although slow changes in the birefringence will be sufficiently canceled by the reciprocal
nature of the Sagnac interferometer, birefringence modulations at frequencies in the
acoustic range of interest will produce polarization noise. Most birefringence modulation
at these frequencies occurs as a result of physical fiber movement Thus, the Sagnac
loop should remain stationary in order to reduce the polarization nolse (as well as
the distributed pick-up signal).
Noise Sources Introduced by the use of the Sagnac interferometer
Thermal Phase Noise
[0045] Because the index of refraction of the fiber changes with temperature, thermal actuations
in a fiber will produce phase fluctuations in the light traveling through it These
index variations ate uncorrelated over the length of fiber, and thus the resulting
phase fluctuations scale as the square root of length. Because Mach-Zehnder interferometers
typically use less than 100 meters of fiber in each arm, the magnitude of this thermal
phase noise is negligible. The Sagnac interferometer has a great deal more fiber in
the interferometer and as a result thermal phase noise can become a Smiting noise
source. The magnitude of this thermal phase noise in a Sagnac interferometer has been
described theoretically and confirmed by experiment. (See, for example, Sverre Knudsen,
et al.,
Measurements of Fundamental Thermal lnduced Phase Fluctuations in the Fiber of a Sagnac
Interferometer, IEEE Photonics Technology Letters. Vol. 7, No. 1, 1995, pp. 90-93; and Kjell Krakenes, et al.,
Comparison of Fiber-Optic Sagnac and Mach-Zehnder Interferometers with Respect to
Thermal Processes in Fiber, JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 13, No. 4, April 1995, pp. 682-686.). For loops greater than 2 km, the thermal
phase noise can exceed
in the frequency range of interest, which is on the order of the required array sensitivity.
[0046] The thermal phase noise can be considered as a source of distributed pick-up noise,
akin to an external modulation to the delay loop, and as such can be reduced by using
empty rungs, as described above. Thermal phase noise can also be reduced by shortening
the loop length. As discussed above, the loop length can be shortened without changing
the low frequency sensitivity by increasing the hydrophone fiber length by the same
factor as that by which the delay loop was deceased. For example a 40-ton delay loop
with 50 meters of hydrophone fiber has the same low-frequency response as a 20-km
delay loop with 100 meters of fiber. The latter combination however will suffer less
thermal phase noise because the total delay loop length is shorter by almost a factor
of two.
Kerr Effect Induced Phase Noise
[0047] Kerr-induced phase shifts which can be generated in a Sagnac interferometer have
received a great deal of attention for the fiber optic gyroscope. (See, for example,
R.A. Bergh, et al.,
Source statistics and
the Kerr effect in fiber-optic gyroscopes,
OPTICS LETTERS, Vol. 7, No. 11, November 1982, pp. 563-565; R.A. Bergh, et al.,
Compensation of the optical Kerr effect in fiber-optic gyroscopes,
OPTICS LETTERS, Vol. 7, No. 6, June 1982, pp. 282-284; and N.J. Frigo, et al.,
Optical Kerr
effect in fiber gyroscopes:
effects of nonmonochromatic sources,
OPTICS LETTERS, Vol. 8, No. 2, February 1983, pp. 119-121.) The demands of the gyroscope and the
acoustic sensor, however, are different because the gyroscope measures DC levels.
Small DC offsets created by Kerr induced phase shifts which would limit a fiber gyroscope
are non-issues with an acoustic sensor. The Kerr-induced DC phase shift is not a problem
as long as it does not move the bias point too far away from quadrature. The intensity
noise on the light source can produce a Kerr induced phase noise on the output. However,
the magnitude of this Kerr-induced AC phase noise is small as long as the Kerr-induced
DC phase shift remains small. The origin of Kerr-induced phase shifts in the Sagnac
sensor array is different than in the fiber gyroscope. The asymmetry of the Sagnac
sensor array invites such a Kerr phase shift much more readily than the nominally
symmetric gyroscope does. That asymmetry results from the array portion as well as
any placement of EDFAs which are asymmetric, in that one beam sees gain before propagating
through the delay loop, then sees loss, while the counter-propagating beam sees loss,
then sees gain. It is possible to balance these asymmetries and null the Kerr-induced
phase shift by choosing the proper location for EDFAs in the delay loop. The specifics
depend on the exact array configuration and which multiplexing scheme is used.
Non-linear phase modulation resulting from the EDFAs
[0048] The population inversions created in the EDFAs induce a phase shift on the signal
light that passes through it. (See, for example, M.J.F. Digonnet, et al.,
Resonantly Enhanced Nonlinearity in Doped Fibers for Low-Power All-
Optical Switching: A Review, OPTICAL FIBER TECHNOLOGY, Vol. 3, No. 1, January 1997, pp. 44-64.) This phenomenon has been used to produce
all-optical interferometric switches. In a Sagnac sensor array, the EDFAs within the
interferometer create a nonlinear phase shift via the same mechanism. Variations in
the population inversion due to pump or signal power fluctuations will produce phase
modulations which will be converted to an intensity noise.
[0049] In order to estimate the magnitude of this noise source, a determination must be
first made as to how the inverted population responds to pump and signal power fluctuations.
This is relatively straightforward to do by invoking the rate equations for an erbium
system:
where N
1 and N
2 are the population densities of the lower and excited states respectively, N
0 is the total population density,
l is the intensity, σ is the cross section, A
eff is the effective mode area in the fiber, and τ
2 is the lifetime of level two. The subscripts
p and
s denote pump and signal, respectively, and the superscripts
a and
e denote absorption and emission, respectively.
[0050] By splitting N
1, N
2, I
p, and l
s into their steady-state and time-varying components, then substituting this into
Equation 12 and combining Equation 12 with Equation 11, the result is:
where the superscript ss denotes steady-state values, and the time-varying components
are now written as explicit functions of time (N
2=N
2ss+N
2(t)). If it is assumed that N
2(t) is much smaller than N
2ss, then the last two terms in Equation 13 can be neglected. By writing I
p(t)=I
pmsin(f
pt) and I
s(t)=I
smsin(f
st) (where
Ipm and I
sm denote the modulation amplitudes of l
p(t) and I
s(t), respectively, and f
p and f
s respectively denote the pump and signal modulation frequencies) and solving the resulting
differential equations, it can be found that
where:
If it is assumed that λ
p=1480 nm, λ
s=1550 nm, and I
pss=1 W, and if typical erbium-silica cross sections are assumed, then Equations 14 and
15 simplify to:
[0051] The pump-induced population inversion fluctuations (Equation 17) will be analyzed
first. If I
sss=
1 mW, |
pss=1 W, and it is assumed that
(120 dB/
electronic SNR), then
at frequencies well below 4.3 kHz. In order to convert this figure to a phase modulation,
the fact that 10 mW of pump power absorbed in an erbium-doped fiber induces approximately
7 radians of phase shift at 1550 nm can be used. (See, for example, M.J.F. Digonnet,
et al.,
Resonantly Enhanced Nonlinearity in Doped Fibers for Low-PowerAll-Opfical Switching:
A Review, OPTICAL FIBER TECHNOLOGY, Vol. 3, No. 1, January 1997, pp. 44-64.) Using simulations, 10 mW of absorbed pump
power in a typical erbium-doped fiber provides approximately 6 dB of small signal
gain at 1550 nm, which is close to the gain required by each amplifier in an array
with distributed EDFAs. (See, for example, Craig W. Hodgson, et al.,
Optimization of Large-Scale Fiber Sensor Arrays Incorporating Multiple Optical Amplifiers-Part
I: Signal-to-Noise Ratio; Craig W. Hodgson, et al.,
Optimization of Large-Scale Fiber Sensor Arrays Incorporating Multiple Optical Amplifiers-Part
II: Pump Power, Jefferson L. Wagener, et al.,
Novel Fiber Sensor Arrays Using Erbium-Doped Fiber Amplifiers; and C.W. Hodgson, et al.,
Large-scale interferometric fiber sensor arrays with multiple optical amplifiers, cited above.) Therefore, each amplifier provides approximately 7 radians of DC phase
shift. Since the nonlinear phase shift is proportional to the upper state population,
N
2, it can be written that ΔN
2/N
2ss=Δφ/φ
ss. Using this relation and Equation 17 again for
and f
s<<4.3 kHz, the low-frequency phase noise induced by each EDFA is
If it is assumed that there are a total of 500 such amplifiers and that the phase
modulations from all 500 amplifiers add coherently, the total pump noise induced phase
shift can be estimated to be
The target phase noise floor is typically set to
indicating that the nonlinear phase-noise induced by the EDFAs due to pump power
fluctuations is close to but not significantly larger than the required phase noise
floor. In practice, the amplifiers' phase modulations will not add coherently, which
will reduce the
figure.
[0052] Calculations of the induced phase shift due to signal power fluctuations are more
complicated because the signal power not only has intensity noise but is also modulated
by the multiplexing scheme. Again considering the TDM case, in general, while a given
pulse is traveling through a particular EDFA, there may or may not be a counter-propagating
pulse traveling through that EDFA at the same time. Taking the worst case in which
there is always a counter-propagating pulse, I
sm is twice the intensity noise of each individual pulse. For the amplifiers, I
sm is typically 1.5 to 2 times the intensity noise of each individual pulse. Assuming
the signal light has an electronic SNR of
at acoustic frequencies
and inserting this figure into Equation 18 along with |
pss=1 W and |
sm=2 mW, it can be calculated that |N
2(f
s)|/N
2ss is approximately
at frequencies much lower than 4.3 kHz and that the phase noise induced by signal
intensity noise in each EDFA is thus
Again assuming 500 amplifiers and coherent addition of all EDFA-induced phase modulation,
the total EDFA induced phase noise on each pulse is
a level which could again limit the performance of the Sagnac sensor array. However,
a more detailed study taking into account the multiplexing scheme and exact timing
of the array is needed for a more accurate calculation.
Multiplexing Schemes in a Sagnac array
Time-Division Multiplexing
[0053] It has been assumed thus far that the Sagnac sensor array is operated in a TDM configuration.
It should be noted that, in the Sagnac sensor array, the source requirements for such
a TDM system are not as demanding as those of a Mach-Zehnder interferometric sensor
array in a TDM configuration. The reason for this is the use of the broadband source
in the Sagnac sensor array. In the Mach-Zehnder interferometric sensor array, the
light from adjacent rungs is coherent due to the narrow linewidth source, and thus
extremely high extinction ratios on the input pulse are required to prevent multi-path
coherent interference. These high extinction ratio requirements are achieved by placing
multiple modulators in series, which results in a complicated, high loss, and expensive
source. In the Sagnac sensor array, the required extinction ratio need not be as high
because the broadband source eliminates any possibility of multi-path coherent interference.
In addition, the narrow linewidths required by the Mach-Zehnder interferometric sensor
array prevent the use of a pulsed laser source in place of a continuous wave (cw)
laser source which is externally modulated with Lithium Niobate intensity modulators.
In the Sagnac sensor array, either a continuous-wave ASE source which is externally
modulated, a pulsed ASE source, or some combination thereof could be used to construct
the source. Again, the reason for this is that the Sagnac sensor array does not require
a narrow linewidth source. Although the present invention does not require a narrow
linewidth source, it should be understood that the Sagnac sensor array of the present
invention can be used with a narrow linewidth source, such as, for example, a laser.
Frequency Division Multiplexing
[0054] The use of the broadband source also allows the Sagnac sensor array to operate in
non-TDM configurations without changing the design or requiring additional sources.
Frequency division multiplexing (FDM) is commonly used with Mach-Zehnder interferometric
sensor arrays using the Phase-Generated Carrier (PGC) scheme but is also compatible
with the Sagnac sensor array. Figure 14 shows a basic Sagnac sensor array 400 using
a FDM scheme. A fiber superfluorescent source (SFS) 402 (or other broadband source,
such as, for example, an LED) generates input light. A chirped intensity modulation
is applied to the input light via an intensity modulator 404 which is controlled by
a chirped frequency generator 406. The modulated light enters a sensor array 410 via
a 3x3 coupler 412. The light passes through a delay loop 414 and plural sensing rungs
416(i) having respective sensors 418(i). Empty rungs (not shown) can also be included
if desired. After passing through the delay loop 414 and the rungs 416(i), the light
exits from the sensor array 410 through the coupler 412 and is detected by a detector
420 which generates an electrical output signal responsive to the detected light.
The electrical output signal from the detector 420 is mixed in a mixer 422 with the
same chirped frequency which has been time delayed by a delay 424 which delays the
chirped frequency by a time Δt. In the setup illustrated in Figure 14, the output
of the mixer 422 is applied to a spectrum analyzer 426. In an operational embodiment,
the output of the mixer 422 is applied to a signal processing subsystem (not shown)
which analyzes the output of the mixer 422 to reproduce the acoustic signals impinging
on the array 410.
[0055] The signals returning from the sensors 418(i) in the various rungs 416(i) are further
delayed with respect to the delayed chirp frequency. This is illustrated by the graphs
in Figure 15 by the original chirped frequency 450, the delayed chirped frequency
452 from the delay 424, the chirped return signal 460 from the first rung, the chirped
return signal 462 from the second rung and the chirped return signal 464 from the
third rung. In the mixer 422, separate beat frequencies f
b1 470, f
b2 472, f
b3 474, respectively (shown in Figure 14), are formed between the mixing chirped frequency
452 and each of the signals returning from the various rungs in the Sagnac sensor
array 410. (See, for example, S.F. Collins, et al.,
A Multiplexing Scheme For Optical Fibre Interferometric Sensors Using An FMCW Generated
Carrier, OFS '92 Conference Proceedings, pp. 209-211.) Although only three chirped return signals 460, 462, 464
are illustrated in Figure 15, it is contemplated that up to N return signals can be
provided, where N is the number of rungs in the array 410. The chirped return signals
from the Nth rung causes a beat frequency f
bN in the mixer 422.
[0056] As illustrated by a pictorial representation of a spectral output in Figure 14, acoustic
modulation of the signals will appear as upper sidebands 480, 481, 482 and lower sidebands
484, 485, 486 to the beat frequencies. An advantage of this FDM scheme is that the
demands on the array timing are greatly relaxed over those required in a TDM system.
A TDM system requires a specific delay between adjacent rungs in order to prevent
pulses from overlapping, and this can present a demanding engineering problem. In
FDM, variations in fiber lengths shift beat frequencies but do not induce overlap
between signals as long as these beat frequencies are separated by twice the acoustic
detection range. The latter is accomplished by selecting the proper chirp rate. Unlike
in a TDM system, all paths return light at all times, which can result in phase noise
between the different incoherent signals. The broadband ASE light source minimizes
the magnitude of this phase noise. (See, for example, Moslehi,
Analysis of Optical Phase Noise in Fiber-Optic Systems Employing a
Laser Source with Arbitrary Coherence Time, Journal of Lightwave Technology, Vol. LT-4, No. 9, September 1986, pp.1334-1351.)
Code Division Multiplexing
[0057] Code division multiplexing (CDM) has received increased attention lately for its
use in sensor arrays. (See, for example, A.D. Kersey, et al.,
Code-division Multiplexed Interferometric Array
With Phase Noise Reduction And Low Crosstalk, OFS '92 Conference Proceedings. pp. 266-269; and H.S. Al-Raweshidy, et al.,
Spread spectrum technique for passive multiplexing of interferometric optical fibre
sensors, SPIE, Vol. 1314 Fibre Optics '90, pp. 342-347.) As illustrated for a Sagnac sensor array
600 in Figure 16, in CDM, the input light from a fiber superfluorescent source 602
(or other broadband source, such as, for example, an LED) is modulated in an intensity
modulator 604 according to a pseudo-random code generated by a code generator 606.
The modulated light is applied to an interferometric loop 608 via a 3x3 coupler 610
and propagates through a delay loop 614 and a plurality of rungs 616(i) in an array
612. In the illustrated embodiment, each rung 616(i) includes a respective sensor
618(i). Empty rungs (not shown) can also be included if desired. The light returns
from the loop via the 3×3 coupler 610 and is detected by a detector 620. The electrical
output of the detector 620 is applied to a correlator 622 along with the output of
the code generator 606, which output is delayed for a duration τ
cor by a delay 624. The bit duration of the pseudo-random code is shorter than the propagation
delay between adjacent rungs in the anay 612 When
τcor is equal to one of the loop travel times τ
i, through a respective rung 616(i), then the signal returning from this sensor in the
rung 616(i) is correlated to the delayed pseudo-random code. The other signals, which
have delays τj where |τ
j-τ
t|>τ
bit correlate to zero. The correlation process involves, for example, multiplying the
detected signal by 1 or - 1 (or gating the signal in an electronic gate 630 to the
non-inverting and inverting inputs of a differential amplifier 632) depending on whether
the correlating code is on or off. The output of the differential amplifier on a line
634 is the correlated output The signal is then time averaged over a period t
avg equal to the duration of the code. The uncorrelated signals time average to zero,
thereby isolating the signal from sensor 618(i). τ
cor is scanned to retrieve sequentially the signals from all sensors.
[0058] An advantage of CDM over TDM is that the delay between sensors does not have to be
controlled accurately. Any loop delays τ
j in which |τ
j-τ
j±1|>τ
bit is acceptable (where τ
bit is the duration of a pulse in the code). Correlating requires a knowledge of the
τj's, which are easily measured. As with FDM, the use of a broadband source benefits
reducing the phase noise which results from the addition of all the signals together.
[0059] The foregoing described a novel design for an acoustic sensor array based on the
Sagnac interferometer. The major advantages of this design are the use of common-path
interferometers. This eliminates the conversion of source phase noise into intensity
noise, which is prevalent in Mach-Zehnder interferometric sensors, and allows the
use of a cheap, high-power ASE source or other broadband source. The response of the
Sagnac sensor array as a function of acoustic frequency is shown to match the ocean
noise floor. The design also allows the dynamic range to be dramatically increased
without adding hydrophones by using one additional, very short delay loop. A technique
for eliminating polarization-induced signal fading was discussed above. The Sagnac
sensor array also allows the use of several multiplexing schemes in a simpler form
than is achievable with a standard Mach-Zehnder array. Because of these features,
the Sagnac sensor array design provides a very promising alternative to Mach-Zehnder-interferometer-based
sensor arrays.
Folded Sagnac Sensor Array
[0060] Figures 17-20 illustrate alternative embodiments of a distributed acoustic sensor
array based upon the Sagnac effect which has an architecture modified to reduce the
distributed pick-up from the downlead fibers. In particular, Figure 17 illustrates
a basic folded Sagnac acoustic fiber sensor array 700 which comprises a source 702,
a first detector 704 and a second detector 706. Preferably, the source 702, the first
detector 704 and the second detector 706 are located in the dry end of the sensor
array 700 (e.g., on shore or on board a ship).
[0061] The source 702 generates light pulses which are coupled to a 3×3 coupler 710 via
a downlead fiber 708. As illustrated, the 3×3 coupler is located in the wet end (e.g.,
proximate to the ocean floor). The 3x3 coupler 710 has a first output port coupled
to one end of a common fiber rung (rung 0) 712, has a second output port coupled to
a first array input/output fiber 714 of an array 716, and has a third output port
which is non-reflectively terminated. Approximately 33 percent of the light from the
source 702 is coupled to each of the first and second ports of the 3x3 coupler and
thus approximately 33 percent of the light propagates to the common fiber rung 712
and approximately 33 percent of the light propagates to the array 716. As discussed
above, although described herein as a 3x3 coupler 710, other n×m couplers (e.g., a
2×2 coupler, a 4×4 coupler, etc.) can be used with the embodiment of Figure 17 and
the alternative embodiments of the present invention described below.
[0062] The array 716 comprises a plurality of rungs 718(i) (i.e., 718(1), 718(2) ... 718(N))
coupled between the first array input/output fiber 714 and a second array input/output
fiber 720. Each rung 718(i) includes a respective acoustic sensor (i.e., hydrophone)
722(i). The array 716 advantageously includes distributed erbium doped fiber amplifiers
(EDFAs) 724, such as described above in connection with Figure 3. (The pump source
for the EDFAs 724 is not shown in Figure 17.) Although described herein with respect
to the array 716, other array configurations can also advantageously be used in the
present invention.
[0063] The second array input/output fiber 720 couples the array 716 to a first port of
a 2×2 coupler 730. A second end of the common rung (rung 0) 712 is coupled to a second
port of the 2×2 coupler 730. Although described herein as an array 716 comprising
plural sensors 722(i), it should be understood that the present invention has applications
for a sensor system having only a single sensor 722.
[0064] A third port of the 2×2 coupler 730 is nonreflectively terminated at a terminal 732.
A fourth port of the 2x2 coupler 730 is coupled to a delay loop downlead fiber 740.
The delay loop downlead fiber 740 couples the fourth port of the 2×2 coupler to a
first end of a delay loop 750. The delay loop 750 may be located either in the dry
end as shown or in the wet end. A second end of the delay loop 750 is coupled to a
reflector 752 such that light exiting the second end of the delay loop 750 is reflected
back into the delay loop 750, propagates through the delay loop 750 and propagates
through the delay loop downlead fiber 740 back to the fourth port of the 2×2 coupler
730. The light returned from the loop downlead fiber 740 is divided by the 2×2 coupler
730 with substantially equal portions propagating in the common rung 712 and in the
array 716 with both portions propagating toward the 3×3 coupler 710. The two portions
are combined in the 3×3 coupler 710 where light pulses which have traveled the same
distance through the array 716 and through the common rung 712 interfere and light
pulses which have traveled different distances do not interfere. The signals resulting
from the interference are output from the 3×3 coupler 710 as first and second output
signals which respectively propagate to the first detector 704 via a first detector
downlead fiber 770 and propagate to the second detector 706 via a second detector
downlead fiber 772. The detectors 704, 706 generate electrical output signals which
are analyzed by electronics (not shown) in a conventional manner to reproduce the
acoustic signals impinging on the sensors 722(i). As discussed below, the signals
which interfere within the 3×3 coupler 710 return from each sensor 722(i) at different
times, and can therefore be separated by time division multiplexing, frequency multiplexing,
code division multiplexing, or the like, as discussed above. The non-interfering signals
do not generate detectable output signals and are ignored.
[0065] The embodiment of Figure 17 can be further modified by inserting a depolarizer (not
shown) in one of the fiber segments 712, 714 or 720 in conjunction with an unpolarized
source, as described above in connection with the Sagnac interferometer. Such embodiments
will be described below in connection with Figures 23A, 23B and 23C.
[0066] The light in a single pulse from the source 702 will now be traced through the sensor
array 700. A source pulse from the source 702 is launched and travels down the source
downlead 708 and through the 3×3 coupler 710 to the common rung 712 and to the array
716. Together, the common rung 712 and the N rungs 718(i) in the array 716 provide
N+1 separate paths for the source pulses to travel to the 2×2 coupler 730. Because
there are N+1 separate paths for the source pulse to travel, the source pulse is split
into N+1 separate pulses which pass through the 2x2 coupler 730 and travel down the
delay loop downlead 740 to the delay loop 750. After passing through the delay loop
750, the N+1 pulses are reflected by the reflector 752 and then propagate back through
the delay loop 750, down the delay loop downlead 740 to the 2×2 coupler 730 in the
wet end, still as N+1 separate pulses. Each of the N+1 pulses is again split into
N+1 pulses in the common rung 712 and the N rungs 718(i). After passing back through
the common rung 712 and the rungs 718(i), the (N+1)
2 pulses are combined in the 3x3 coupler 710 and then return down the detector downleads
770, 772 back to the dry end where the pulses are detected by the first and second
detectors 704, 706 and analyzed.
[0067] Because there are (N+1)
2 possible separate combinations of paths from the source 702 to the reflector 752
and back to the detectors 704, 706, there are (N+1)
2 returned pulses. The only pulses that will interfere in a useable manner are pairs
of pulses which travel the same exact path length but in opposite order. For the purposes
of the following discussion, a pulse will be identified by two numbers where the first
number identifies the path taken by the pulse from the source 702 to the reflector
752, and the second number identifies the path taken by the pulse from the reflector
752 back to the detectors 704, 706. For example, the pulse 0,1 travels through the
common rung (rung 0) 712, then through the delay loop 750, to the reflector 752, back
through the delay loop 750, and then through rung 718(1). The pulse 1,0 travels first
through the rung 718(1), then through the delay loop 750, to the reflector 752, back
through the delay loop 750, and then through the common rung (rung 0) 712. Because
the distance traveled by the pulse 0,1 is identical with the distance traveled by
the pulse 1,0, the pulse 0,1 and the pulse 1,0 interfere when combined at the 3x3
coupler 710 and therefore define a common-path interferometer (i.e., a folded Sagnac
interferometer) in the same manner as the Sagnac interferometers described above.
Acoustic sensing results from the hydrophone 722(1) which is placed in rung 1 which
responds to acoustic modulation. The interfering pulses 0,1 and 1,0 see the hydrophone
722(1) at different times and thus pick-up a phase difference due to the time varying
acoustic modulation of the hydrophone 722(1). At the 3x3 coupler 710, this phase difference
is converted into an intensity modulation which is transmitted down the detector downleads
770, 772 to the detectors 704, 706. The same effect occurs for the pulses 0,2 and
2,0, for the pulses 0,3 and 3,0, etc.
[0068] Because the folded Sagnac interferometer is common-path, the source 702 can have
a short coherence length, which means that interference will only occur between pulses
which have traveled nearly identical paths. Therefore, pulse i,j will interfere with
pulse j,i only. As stated above, there are N interferometers of interest (pulse 0,i
interfering with pulse i,0 for i=1 to N). There are also the many other interferometers
which do not include the common rung (rung 0) 712 (e.g., pulse 1,2 interfering with
pulse 2,1, pulse 1,3 interfering with pulse 3,1, etc.). Such interfering pulses contribute
noise to the useful pulses, and shall be referred to herein as noise pulses. These
noise pulses carry two types of noise. As with all pulses, they carry additional shot
noise, ASE-signal beat noise (in an amplified array), phase noise, etc., which increase
the detected noise. The noise pulses which form an unwanted interferometer (pulse
1,2 interfering with pulse 2,1, etc.) also carry intensity modulation due to interferometric
sensing of acoustic waves. This intensity modulation is an unwanted signal and can
be viewed as a source of noise. It is important to note that these unwanted interferometers
have as their interfering point couplers 280(1) through 280(N) where the rungs 218(1)
through 218(N) couple to the first input/output fiber 714 of the array 716, whereas
the signal pulses interfere at the 3×3 coupler 710. Because the noise pulses interfere
before they reach the 3×3 coupler 710 coupler, the intensity modulation of the noise
pulses is provided symmetrically to both detectors 704 and 706. The signal pulses
which interfere at the 3x3 coupler 710 however produce an asymmetric intensity modulation.
Therefore, by differentially amplifying the currents from the detectors 704, 706,
the intensity modulation of the signal pulses adds and the intensity modulation of
the noise pulses subtracts, thus reducing the noise contribution of the unwanted interferometers.
[0069] To completely eliminate all the noise added by these noise pules, the pulses of interest
can be separated from the noise pulses by using a time division multiplexing scheme
and properly choosing delay lengths. In particular, the optical path length from the
3×3 coupler 710 through the common rung 712 to the 2×2 coupler 730 is selected to
correspond to a propagation time τ. The optical path length of a fiber portion from
the 3×3 coupler to the coupler 780(1), through the first rung 718(1), to a corresponding
coupler 790(1) and to the 2×2 coupler 730 is selected to be (N+1)τ. A portion of the
optical path length is a common path from the 3×3 coupler 710 to the coupler 780(1)
and from the coupler 790(1) to the 2×2 coupler 730, and a portion of the optical path
length is through the rung 718(1). The optical path lengths through each of the rungs
718(i) are preferably selected to be approximately equal. The total length of the
optical path from the coupler 780(1) to the coupler 780(2) and the optical path from
a coupler 790(2) to the coupler 790(1) is selected to be τ such the total optical
path length from the 3×3 coupler 710 to the 2×2 coupler 730 through the second rung
718(2) is τ longer than the total optical path length from the 3×3 coupler 710 to
the 2×2 coupler 730 through the first rung 718(1) (i.e., the total optical path length
between the two couplers 710, 730 through the second rung 718(2) is (N+2)τ). The total
additional optical path length for each successive is selected to be τ. Thus, the
travel time of light from the 3×3 coupler 710 through a rung 718(i) to the 2×2 coupler
730 is defined as the delay time T
i of the rung 718(i).
[0070] In accordance with the foregoing description, T
i is determined by the optical path lengths through the rungs as follows:
i = 0 (for the common rung 712)
[0071] 1 ≤ i ≤N (for each of the sensing rungs 718(1), 718(2), etc. From the foregoing,
it can be seen that the optical path length through the farthest rung N is (N+N)τ
or 2Nτ.
[0072] The duration of each pulse is selected to be no more than τ. Thus, as illustrated
in Figure 18, the first pulse 800 returned to the 3x3 coupler 710 will be the pulse
which traveled through the common rung 712 (i.e., rung 0) from the source 702 to the
reflector 752 and back to the detectors 704, 706. This pulse has a total propagation
time of 2τ. (In comparing propagation times, the propagation time of each pulse to
the reflector 752 through the delay loop 750 and back is ignored because the propagation
time is common to all pulses and simply operates as an offset (not shown) to the timing
diagram in Figure 18.) The next set 810 of pulses returned to the detectors 702, 706
are the pulses which travel through the common rung 712 in one direction and travel
through a sensing rung 718(i) in the opposite direction (i.e., the pulses 0,1 and
1,0; 0,2 and 2,0; 0,3 and 3,0, through 0,N and N,0). These pulses have respective
propagation times of 2τ+Nτ, 3τ+Nτ, 4τ+Nτ, through (N+1)τ+Nτ. Thus, all the useful
pulses are received between a time (N+2)τ and a time (2N+2)τ (including the duration
τ of the last pulse received). In contrast, the interfering pulses which travel through
a sensing rung 718(i) in both directions (i.e., the pulses, 1,1, 1,2 and 2,1, 1,3
and 3,1 ... 2,2, 2,3 and 3,2, ... etc.) are received as a set of pulses 820 between
a time 2(N+2)τ and a time (4N+1)τ. Thus, the signal pulses are separated from the
noise pulses.
[0073] For example, in Figure 18, the number of returned pulses as a function of time is
plotted for N=50. As illustrated, a single pulse is received at a time 2τ. Thereafter,
no pulses are received during the interval 3τ through 52τ. Then, from 52τ through
102τ, two pulses are received during each time interval. The noise pulses then return
from a time 102τ to a time 201τ. In this way, the signal pulses are separated in time
from the noise pulses, thus preventing the noise pulses from adding noise to the signal
pulses. The electronics (not shown) are readily synchronized to only look at the pulses
received between the time 52τ and the time 102τ.
[0074] It should be noted that the source 702 can be activated to send out the next pulse
at the at a time interval of 150τ relative to the previous pulse because the 0τ to
50τ interval in response to the next pulse can overlap the 150τ to 200τ interval of
noise pulses returning in response to the previous source pulse. Thus, a next set
830 of useful pulses can begin arriving at a time 201. Therefore, the embodiment of
Figures 17 and 18 has an overall duty cycle of roughly 1/3 for useable signal information.
[0075] The advantage of the folded Sagnac acoustic fiber sensor 700 over the Sagnac loop
illustrated in the previous figures is that the delay fiber 750 is insensitive to
modulation. Because the downleads are often quite long and are subjected to large
movements and vibrations, distributed downlead pickup is a potentially serious limitation
to a Sagnac acoustic fiber sensor. In the folded Sagnac acoustic fiber sensor 700,
the source 708 and detector downleads 770, 772 are insensitive because they occur
outside the interferometer. The delay loop downlead 740 is insensitive because all
the interfering pulses travel this same fiber separated by small time delays (approximately
1 microsecond) and thus see the same perturbations. Any low frequency (much less than
approximately 1 MHz) modulation to the delay loop downlead and delay loop itself it
seen substantially equally by both interfering pulses and thus does not contribute
to a phase difference. The array portion 716 and the common rung 712 comprise the
only sensitive fibers in the interferometer 700.
[0076] As shown in Figure 17, the remotely pumped distributed erbium doped fiber amplifiers
(EDFAs) 724 can be located throughout the array 216 to regenerate power, as discussed
above.
[0077] The 3×3 coupler 710 is used to passively bias each sensor 722(i) near quadrature
and to allow source noise subtraction. Noise subtraction results from the fact that
each detector 704, 706 is biased on an opposite slope (because of the way the signals
coming out of the 3×3 coupler 710 are phased with respect to each other), causing
phase modulation to asymmetrically affect the intensity at each detector, while source
excess noise symmetrically affects the intensity at each detector. Therefore, by differentially
amplifying the detector outputs, the phase modulation induced intensity variations
are added and the source's intensity noise is subtracted in the same manner that the
signals from the unwanted interferometers would be subtracted.
[0078] It should be understood with respect to Figures 17 and 18 that a similar time divisional
multiplexing effect can be accomplished by providing a longer optical path length
through the common rung 712 and shorter optical path lengths through the sensing rungs
718(i). For example, the common rung 712 can advantageously be selected to have an
optical path length of 2Nτ (i.e., T
0 = 2N), and the optical paths through the rungs can advantageously be selected to
be τ, 2τ, 3τ,... Nτ. The foregoing can be summarized as:
i = 0 (for the common rung 712)
[0079] 1 ≤ i ≤ N (for each of the sensing rungs 718(1), 718(2), etc.
[0080] Thus, the first signal to return will have an optical propagation time (again subtracting
out the propagation time through the delay loop 750 which is common to all signals)
of 2τ which is the time required to pass through the first rung 718(1) in both directions.
The longest delay of any signal which passes through one of the sensing rungs 718(i)
in both directions is 2N for a signal pulse which travels both directions through
the farthest sensing rung 718(N). The first useable signal to return is a signal which
results from the interference of a signal which travels in to the reflector 752 through
the common rung 712 and returns through the first sensing rung 718(1) with a signal
which travels to the reflector 752 through the first sensing rung 718(1) and returns
through the common rung 712. The interference signal will arrive at a time (2N+1)τ
which is later than the last unwanted signal. The last useable signal will arrive
at a time (2N+N)τ (i.e., 3Nτ). Finally, a signal produced by a pulse which traveled
to and from the reflector 752 in the common rung 712 arrives at a time 4Nτ, which
is well separated from the useable interference signals.
[0081] It is desirable for acoustic sensors to have as large a dynamic range (range of detectable
acoustic modulation amplitudes) as possible. Without using demodulation techniques
such as the phase-generated carrier scheme, the minimum detectable phase modulation
is set by the noise performance of the array, and the maximum detectable phase modulation
(approximately 1 rad) is set by the nonlinear response function of an interferometer.
In a Mach-Zehnder sensor, the mapping of acoustic modulation to phase modulation is
a function of only the hydrophone's responsivity. Thus, these limits on the detectable
phase modulation along with this mapping of acoustic modulation into phase modulation
give the range of acoustic modulation the sensor can detect.
[0082] In a folded Sagnac acoustic fiber sensor array, the mapping of acoustic modulation
into phase modulation is a function of both the responsivity of each of the hydrophones
(sensors) 722(i) and the length of the delay loop 750. Thus by changing the length
of the delay loop 750, the dynamic range of the sensors 722(i) can be adjusted without
modifying the hydrophones 722(i) themselves. In addition, if two reflectors 742(1)
and 752(2) are used, each sensor 718(i) can have two different delay loops 750(1)
and 750(2), as shown in a sensor 850 in Figure 19. This allows each sensor 722(i)
to return two signals which have different dynamics ranges, as discussed above with
respect to Figures 7 and 8, thereby greatly increasing the total dynamic range of
each sensor 722(i). The penalty is a reduction in duty cycle for each individual signal
by a factor of 1/(number of delay loops).
[0083] Figure 20 illustrates a sensor 900 which implements a phase-nulling technique similar
to techniques which have been used in fiber gyroscopes. The delay loop reflector 752
of Figure 17 is not used in the sensor 900 of Figure 20. Rather, the pulses are instead
returned via a return downlead 910 into the previously unused port of the 2x2 coupler
730. An optical isolator 912 is inserted in the return downlead 910 to prevent light
from traveling the delay loop 750 in both directions. The sensor 900 of Figure 20
behaves identically to the sensor 700 of Figure 17 with the reflector 752. However,
the sensor 900 allows the addition of a phase modulator 920 to be inserted into the
return downlead 910. The phase modulator 920 is activated to add a phase shift to
each pulse individually. By feeding the detected phase shift into the phase modulator
920 via a differential amplifier 922, phase changes are nulled out, and the required
applied phase shift in the phase modulator 920 becomes the signal. In this phase nulling
method, the dynamic range of the array 900 is limited only by the maximum phase shift
that the phase modulator 920 can provide.
[0084] Figure 21 illustrates a further alternative embodiment of Figure 19 in which the
two delay loops 750(1) and 750(2) are not connected to the same delay loop downlead.
Rather, the first end of the first delay loop 750(1) is connected to a first delay
loop downlead 740(1) which is connected to the fourth port of the 2x2 coupler 730
as in Figure 19. The second end of the first delay loop 750(1) is coupled to the first
reflector 752(1) as before. The first end of the second delay loop 750(2) is coupled
to the third port of the 2x2 coupler 730 via a second delay loop downlead 740(2),
and the second end of the second delay loop 750(2) is coupled to the second reflector
752(2). Approximately half the light from the 2x2 coupler 730 is coupled to each of
the downleads 740(1), 740(2). The light in each downlead 740(1), 740(2) is delayed
in the respective delay loop 750(1), 750(2) and is reflected back to the 2×2 coupler
730 as before. The reflected light is coupled to the common rung 712 and to the array
716. The delays of the delay loops 750(1), 750(2) are selected so none of the N+1
pulses which propagate from the fourth port of the 2x2 coupler 730 through the first
delay loop 750(1) overlap in time with any of the N+1 pulses which propagate from
the third port of the 2x2 coupler 730 through the second delay loop 750(2). Thus,
the embodiment of Figure 21 provides, similar functionality to the embodiment of Figure
19; however, the embodiment of Figure 21 utilizes the light which was coupled out
of the third port of the 2×2 coupler 730 in Figure 19 and discarded.
[0085] Figure 22 illustrates an alternative embodiment of a fiber optic acoustic sensor
system 1000 using a folded Sagnac sensor array. In the system 1000, a source 1004
is coupled to a first port of a 2×2 polarization maintaining coupler 1006 by an X-polarizer
1008. A detector 1002 is connected to a second port of the 2×2 coupler 1006 via a
X-polarizer 1010. A second detector (not shown) may advantageously be included in
the embodiment of Figure 22 by coupling light from the fiber leading to the source
1004. The X-polarizer 1008 only passes light from the source 1004 having a first polarization
(e.g., an X-polarization). Thus, the polarization maintaining coupler 1006 receives
light having an X-polarization from the source 1004 and couples the light to a common
rung 1020 via a third port and to a sensor array 1022 via a fourth port. The sensor
array 1022 has a similar structure to the sensor array 716 of Figure 17, and like
elements have been numbered accordingly.
[0086] Note that the two X-polarizers 1008, 1010 can be replaced by one or more X-polarizers
in alternative locations in the system 1000.
[0087] The common rung 1020 is coupled via an X-polarizer 1030 to a first port of a second
polarization maintaining 2x2 coupler 1032. The light propagating to the array 1022
first passes through a depolarizer 1034 and then to the first input/output fiber 714.
The depolarizer 1034 couples substantially equal amounts of the X polarized light
to Y polarized light and to Y polarized light. Thus, approximately 50 percent of the
light propagates in the array 1022 as X-polarized light, and approximately 50 percent
propagates in the array 1022 as Y-polarized light.
[0088] After passing through the rungs of the array 1022, the light propagates via the second
input/output fiber 720 and a Y-polarizer 1040 to a second port of the second coupler
1032. The Y-polarizer 1040 allows only Y-polarized light to enter the second coupler
1032. The coupler 1032 combines the light from the array 1022 and from the common
rung 1020 Approximately half the light entering the coupler 1032 is coupled via a
third port of the coupler 1032 to a light absorbing termination 1042, and approximately
half of the light is coupled to a downlead fiber 1050 which propagates the light to
a first end of a delay loop 1052.
[0089] Light passes through the delay loop 1052 to a Faraday rotating mirror (FRM) 1054.
The operation of the Faraday rotating mirror 1054 is well known and will not be described
in detail. Basically, when light is incident onto the Faraday rotating mirror 1054
in one polarization, it is reflected in the orthogonal polarization. Thus, the X-polarized
light which passed through the common rung 1020 is reflected as Y-polarized light,
and the Y-polarized light which passed through the array is reflected as X-polarized
light.
[0090] The reflected light passes back through the delay 1052 and enters the fourth port
of the coupler 1032. The light is coupled to the common rung 1020 and to the array
1022. The X-polarizer 1030 in the common rung passes only the light in the X-polarization
which originally propagated through the array 1022. Similarly, the Y-polarizer 1040
in the array 1022 passes only Y-polarized light which originally propagated through
the common rung 1020.
[0091] After propagating through the array 1022, the returning Y-polarized light is depolarized
in the depolarizer 1034 to produce both X-polarized light and Y-polarized light. The
light from the common rung 1020 enters the third port of the coupler 1006, and light
from the depolarizer 1034 enters the fourth port of the coupler 1006. The light combines
in the coupler, and the X-polarized light from the two ports which has traveled the
same optical distance interferes and is coupled to the first and second ports. The
portion coupled to the second port propagates through the X-polarizer 1010 to the
detector 1002 where the interfering signals are detected.
[0092] It should be understood that only the light which originally traveled different paths
to and from the Faraday rotating mirror 1054 interferes at the coupler 1006. The only
light allowed to propagate through the common rung 1020 in the reflected direction
is X-polarized light which originally propagated in the array 1022 as Y-polarized
light. Similarly, the only light allowed to propagate through any of the rungs of
the array 1022 in the reflected direction is Y-polarized light which originally propagated
in the common rung 1020 as X-polarized light. Potentially interfering light cannot
travel through the rungs in both directions to produce the noise signals described
above in connection with the above-described embodiments. Thus, each of the pulses
generated in the array 1022 from the reflected pulse that originally traveled in the
common rung 1020 can interfere with only a single one of the pulses which was originally
generated in the array 1022 and which propagated in the common rung 1020 after it
was reflected. Thus, it is not necessary in the embodiment of Figure 22 to include
additional delays to separate the useable signal pulses from noise pulses.
[0093] Figures 23A, 23B and 23C illustrate further alternative embodiments of the present
invention. A sensor array 1100 in the embodiments of Figures 23A, 23B and 23C is similar
to the sensor array 700 in the embodiment of Figure 17, and like elements have been
numbered accordingly. The embodiments of Figures 23A, 23B and 23C include an unpolarized
source 1102. The 2x2 coupler 730 of Figure 17 is replaced with a polarization beam
splitter (PBS) 1104 in Figures 23A, 23B and 23C. The use of the polarization beam
splitter 1104 saves approximately 6 dB of power compared to the coupler 730 in Figure
17 and the coupler 1130 in Figure 22. The reflector 752 in Figure 17 is replaced with
a Faraday rotating mirror (FRM) 1106, which is similar to the Faraday rotating mirror
1054 of Figure 22. The 3x3 coupler 710 in Figures 23A, 23B and 23C does not have to
be a polarization maintaining coupler.
[0094] Each of Figures 23A, 23B and 23C includes a depolarizer 1110. In Figure 23A, the
depolarizer 1110 is located on the first array input/output fiber 714. In Figure 23B,
the depolarizer 1110 is located on the common rung 712. In Figure 23C, the depolarizer
1110 is located on the second array input/output fiber 720.
[0095] In the embodiment of Figure 23A, light from the unpolarized source 1102 enters the
3x3 coupler 710 and is coupled in approximately equal portions to the common rung
712 and to the first array input/output fiber 714. As discussed above in connection
with Figures 3 and 17, the use of the 3x3 coupler provides passive biasing near quadrature.
The light propagating in the first array input/output fiber 714 passes through the
depolarizer 1110, which has the effect of causing substantially half of the light
entering the array in one polarization (e.g., the X-polarization) to be coupled into
the orthogonal polarization (e.g., the Y-polarization), and likewise half of the light
entering the array in the Y-polarization to be coupled to the X-polarization. Thus,
after the depolarizer 1110, half of the light in the X-polarization originated in
the X-polarization and the other half of the light in the X-polarization originated
in the Y-polarization. Likewise, after the depolarizer 1110, half of the light in
the Y-polarization originated in the Y-polarization and the other half of the light
in the Y-polarization originated in the X-polarization. Effectively, the depolarizer
1110 scrambles the unpolarized light.
[0096] The light passes through the array 716 in the manner described above in connection
with the other embodiments. The light exiting the array 716 propagates through the
second array input/output fiber 720 to a first port 1121 of the polarization beam
splitter 1104. The polarization beam splitter 1104 splits the incident light into
the two orthogonal polarizations (i.e., the X-polarization and the Y-polarization).
For the purpose of this discussion, it is assumed that the polarization beam splitter
1104 operates like a polarization-dependent mirror oriented at 45°, wherein light
entering the first port 1121 in one polarization (e.g., the X-polarization) is reflected
to a second port 1122 and light entering the first port 1121 in the other polarization
(e.g., the Y-polarization) is transmitted to a third port 1123. In the embodiment
shown, the light exiting the second port 1122 is non reflectively absorbed by the
terminator 732. The Y-polarized light exiting the third terminal 1123 propagates through
the delay loop downlead fiber 740, through the delay loop 750 to the Faraday rotating
mirror 1106. Note that this Y-polarized light from the array portion 716 traveled
through the depolarizer 1110 and half of it was originally X-polarized light and half
of it was originally Y-polarized light. As discussed above, the Faraday rotating mirror
1106 causes the incident light to be coupled to the orthogonal polarization. Thus,
the Y-polarized light is coupled to the X-polarization.
[0097] The X-polarized light reflected by the Faraday rotating mirror 1106 passes through
the delay loop 750 and the delay loop downlead fiber 740 back to the third port 1123
of the polarization beam splitter. Because the light is now in the X-polarization,
the light is reflected to a fourth port 1124 rather than being transmitted to the
first port 1121. Thus, the Y-polarized light which was originally incident on the
polarization beam splitter from the array 716 is coupled to the common rung 712 to
propagate back to the 3x3 coupler 710 in the X-polarization.
[0098] Unpolarized light which propagates from the 3×3 coupler 710 to the polarization beam
splitter 1104 via the common rung 712 enters the polarization beam splitter 1104 via
the fourth port 1124. The components of the light in the Y-polarization are transmitted
to the second port 1122 and are nonreflectively terminated by the terminator 732.
The components of the light in the X-polarization are reflected to the third port
1123 and propagate to the Faraday rotating mirror 1106 via the delay loop downlead
fiber 740 and the delay loop 750. (The reason for including the depolarizer 1110 can
now be understood. Because only the X-polarized light from the common rung 712 is
coupled to the delay loop downlead fiber 740, the depolarizer 1110 ensures that the
light coupled from the array 716 to the delay loop downlead fiber 740 also includes
some light which was originally X-polarized.) The Faraday rotating mirror 1106 reflects
the light as Y-polarized light, and the Y-polarized light propagates through the delay
loop and the downlead fiber to the third port 1123 of the polarization beam splitter
1104.
[0099] The Y-polarized light incident on the third port 1123 of the polarization beam splitter
1104 is transmitted to the first port 1121 and thus to the second array input/output
fiber 720. The Y-polarized light propagates through the array 716 to the first array
input/output fiber 714 and then passes through the depolarizer 1110 to the 3x3 coupler
710. The depolarizer 1110 operates to convert approximately 50 percent of the Y-polarized
light to X-polarized light. The X-polarized light from the depolarizer 1110 interferes
with the X-polarized light from the common rung 712. The resulting combined light
is detected by the detector 704 or the detector 706 in accordance with the phase relationship
between the interfering light signals in the 3x3 coupler 710.
[0100] Note that the X-polarized light incident on the 3x3 coupler 710 from the depolarizer
1110 and the X-polarized light from the common rung 712 travel identical path lengths.
For example, light which propagates through the common rung 712 first, propagates
in the X-polarization through the common rung 712 and then propagates through the
array 716 in the Y-polarization. On the other hand, the light which propagates through
the array 716 first propagates in the Y-polarization through the array 716 and then
propagates in the X-polarization through the common rung. Because the two "counterpropagating"
light signals are in the same polarizations when propagating through the corresponding
portions of the interferometric path, the propagation lengths are identical except
for the effect of incident noise sensed by the array 716.
[0101] It should be understood that the terminator 732 coupled to the second port 1122 of
the polarization beam splitter 1104 can be replaced with a second delay loop (not
shown) and a second Faraday rotating mirror (not shown) to provide a second interferometric
path for light which interferes in the Y polarization. By adjusting the delay provided
by the second delay loop, the return signals from the second interferometric path
can be precluded from overlapping with the return signals from the first interferometric
path.
[0102] The embodiment of Figure 23B is similar to the embodiment of Figure 23A except that
the depolarizer 1110 is positioned in the common rung 712. The effect of the depolarizer
1110 in Figure 23B is (1) to cause a portion of the light in the common rung 712 returning
from the polarization beam splitter 1104 in a single polarization (e.g., the X-polarization)
to be coupled to the orthogonal polarization and (2) to scramble the unpolarized light
which travels from the 3x3 coupler 710 through the common rung 712 towards the polarization
beam splitter 1104. This ensures that the light interferes when it recombines at the
3x3 coupler 710 (the same reason the depolarizer 1110 was added to the fiber 714 of
Figure 23A).
[0103] The embodiment of Figure 23C is also similar to the embodiment of Figure 23A except
that the depolarizer 1110 is positioned in the second array input/output fiber 720.
The embodiment of Figure 23C is functionally equivalent to the embodiment of Figure
23A because it does not matter whether the light passes through the array portion
716 and then passes through the depolarizer 1110 or passes through the depolarizer
1110 and then passes through the array portion 716. Thus, the function of the embodiment
of Figure 23C is substantially the same as the function of the embodiment of Figure
23A, as described above.
[0104] Figure 24 illustrates a further alternative embodiment of the present invention in
which a folded Sagnac sensor array 1200 includes the polarization beam splitter (PBS)
1104, the Faraday rotating mirror (FRM) 1106, and the depolarizer 1110 connected as
shown in the array 1100 in Figure 23A. Other components from Figure 23A are also numbered
as before. Unlike the array 1100 in Figure 23A which has the 3×3 coupler 710, the
folded Sagnac sensor array 1200 has a polarization maintaining (PM) 2×2 coupler 1220
connected in like manner as the 2×2 coupler 1006 in Figure 22. One port of the 2×2
coupler 1220 is connected to a first port of an optical circulator 1222 via a first
polarizer 1224. A second port of the optical circulator 1222 is connected to a first
detector 1226. A third port of the optical circulator 1222 is connected to an unpolarized
source 1228 (e.g., an intensity modulated fiber superfluorescent source). A second
port of the 2×2 coupler 1220 is connected to a second detector 1230 via a second polarizer
1232. The detectors 1226 and 1230 and the unpolarized source 1228 are connected to
the circulator 1222 by standard (not polarization maintaining) fibers. The polarizers
1224 and 1232 are coupled to the polarization maintaining coupler 1220 via the polarization
maintaining fiber such that the polarizers 1224; 1232 are aligned with a same axis
of the polarization maintaining 2×2 coupler 1220. Alternatively, if a polarized source
is used in place of the unpolarized source 1228, the polarized source (not shown)
is connected to a polarization maintaining circulator (not shown) by polarization
maintaining fiber, and the polarization maintaining circulator is connected to the
polarizer 1224 by polarization maintaining fiber. The polarization maintaining components
are connected such that the polarized light from the source passes through the polarizer
1224. The connections from the polarization maintaining circulator to the detectors
1226 and 1230 are provided by standard (not polarization maintaining) fibers.
[0105] The folded Sagnac sensor array 1200 further includes a non-reciprocal phase shifter
1250. The phase shifter 1250 is coupled to the common rung 712 via a first optical
fiber 1252 having a first end 1254 and a second end 1256 and via a second optical
fiber 1258 having a first end 1260 and a second end 1262. The first end 1254 of the
first optical fiber 1252 is coupled to the common rung 712 proximate to the 2x2 coupler
1220 via a first coupler 1264. The first end 1260 of the second optical fiber 1258
is coupled to the common rung 712 proximate to the polarization beam splitter 1104
via a second coupler 1266. The respective second ends 1256,1262 of the first and second
optical fibers 1252, 1258 are coupled to the phase shifter 1250, as discussed below
in connection with Figures 25 and 26.
[0106] Preferably, the common rung 712, the first fiber 1252 and the second fiber 1258 are
polarization maintaining (PM) fibers, and the first coupler 1264, the second coupler
1266 and the 2x2 coupler 1220 are polarization maintaining (PM) couplers. Also, preferably,
the first coupler 1264 and the second coupler 1266 are 50/50 couplers which couple
approximately 50 percent of the light entering the common rung 712 in either direction
to the phase shifter 1250 while approximately 50 percent of the light remains in the
common rung. Thus, the non-reciprocal phase shifter 1250 and the associated fibers
form a second rung 1268 in parallel with the common rung 712.
[0107] Preferably, one of the rungs 712,1268 (e.g., the common rung 712) includes a delay
element (e.g., a delay loop 1269) that introduces a time delay in one rung sufficient
to prevent the pulses propagating through the rungs from overlapping. Thus, the light
returning to 2×2 coupler 1220 from the sensor array 716 comprises two pulses for each
sensor that are spaced apart in time from each other. One pulse comprises the combined
light that passes through the common rung 712 in each direction. The other pulse comprises
the combined light that passes through the non-reciprocal phase shifter 1250 in each
direction. It should be understood that the light pulse that passes through the phase
shifter 1250 in one direction and the light pulse that passes through the common rung
712 in the other direction have substantially different propagation times and will
not overlap in the coupler 1220. Thus, they will not interfere.
[0108] The light that passes though the common rung 712 in one direction does not undergo
any phase shift within the common rung 712 relative to the light that passes through
the common rung in the other direction. Thus, the combined light that passes through
the common rung 712 in both directions has a relative phase bias of zero. However,
as discussed below, the non-reciprocal phase shifter 1250 does introduce a shift of
the light in one direction with respect to the light in the other direction. In particular,
in a preferred embodiment, the phase shifter 1250 introduces a relative π/2 phase
shift between the light in the two directions. Thus, the light entering the coupler
1220 that has propagated through the phase shifter 1250 in both directions will combine
in the coupler 1220 with a π/2 phase bias.
[0109] One skilled in the art will appreciate that the 50 percent coupler 1220 in the interferometric
configuration shown in Figure 24 couples returning light to the output port corresponding
to the original input port when the returning light at the two input ports interferes
in the coupler and has a relative phase difference of 0, 2π, 4π, etc., and couples
returning light to the other output port when the light has a relative phase difference
of π, 3π, 5π, etc. When the returning light has a relative phase difference that is
not a multiple of π, a portion of the returning light is output from both ports. For
example, when the relative phase difference is an odd multiple of π/2 (e.g., π/2,
3π/2, etc.), approximately 50 percent of the returning light is coupled to each output
port. By providing two independent propagation paths, each detector 1226, 1230 receives
two signals that are spaced apart in time and can therefore be separately detected.
One signal has a 0 phase bias, and one signal has a π/2 phase bias so that when one
signal is least sensitive to perturbation, the other signal is most sensitive to perturbation,
and vice versa. It should be understood that additional rungs in parallel with the
common rung 712 and with differing amounts of relative phase shift can be included
to provide pulses with different phase biasing.
[0110] Figure 25 illustrates an alternative configuration of a folded Sagnac sensor array
1200', which is substantially similar to the folded Sagnac sensor array 1200 of Figure
24. In the folded Sagnac sensor array 1200' of Figure 25, the depolarizer 1110 is
located in the second array input/output fiber 720 rather than in the first array
input/output fiber 714. Because of the reciprocal structure of the sensor array 716,
the relocation of the depolarizer 1110 to the fiber 720 does not change the overall
operation of the folded Sagnac sensor array 1200' with respect to the operation of
the folded Sagnac sensor array 1200. Thus, the operation of the folded Sagnac sensor
array 1200' will not be described in detail herein.
[0111] The embodiments of Figures 24 and 25 include the sensor array 716, which was described
in detail above. It should be understood that other configurations of amplified sensor
arrays can also be used in place of the sensor array 716 in the embodiments of Figures
24 and 25.
[0112] Figure 26 illustrates a first preferred embodiment of the non-reciprocal π/2 phase
shifter 1250 of Figures 24 and 25. As illustrated in Figure 26, the phase shifter
1250 comprises a first collimating lens 1270, a first 45° Faraday rotator 1272, a
quarter-wave plate 1274, a second 45° Faraday rotator 1276, and a second collimating
lens 1278. In the illustrated embodiment, the first Faraday rotator 1272, the second
Faraday rotator 1276 and the quarter-wave plate 1274 comprise bulk optic devices that
are commercially available, but may advantageously comprise fiber optic or other waveguide
devices. The collimating lenses 1270, 1278 are positioned proximate to second ends
1256,1262 of the PM fibers 1252,1258 to focus light from the fiber ends 1256, 1262
onto the Faraday rotators 1272, 1276, respectively, and to focus light from the Faraday
rotators 1272, 1276 into the fiber ends 1256, 1262. Each of the Faraday rotators 1272,1276
operates in a well-known manner to cause light input to the Faraday rotator with its
polarization at a particular angle to have the polarization rotated so that the polarization
is at a new angle rotated by a predetermined amount with respect to the original angle.
For example, in the preferred embodiment, each Faraday rotator 1272, 1276 rotates
the polarization of the incident light by 45° in the counterclockwise (ccw) direction.
Thus, as illustrated in Figure 26, light emitted from the end 1256 of the PM fiber
1252 having its polarization oriented horizontally, will be rotated by 45° counterclockwise
in the first Faraday rotator 1272 such that the polarization is oriented at an angle
of 45° in the clockwise direction with respect to the original orientation when it
emerges from the first Faraday rotator 1272.
[0113] The quarter-wave plate 1274 is positioned between the two Faraday rotators 1272,
1276. The quarter wave plate 1274 has a first birefringent axis 1280 and an orthogonal
second birefringent axis 1282. Light propagating in a polarization oriented along
one birefringent axis (e.g., the first birefringent axis 1280) has a slower propagation
velocity than the light propagating in a polarization oriented along the other birefringent
axis (e.g., the second birefringent axis 1282). The quarter-wave plate 1274 is oriented
so that the first birefringent axis 1280, for example, is oriented at 45° in the clockwise
direction to the vertical, and is therefore oriented so that the light emerging from
the first Faraday rotator 1272 is oriented along the first birefringent axis 1280
and is orthogonal to the second birefringent axis 1282. Because of the difference
in propagation velocities along the two axes, the quarter wave plate 1274 introduces
a π/2 or 90° phase shift in the light polarized along the first birefringent axis
1280 with respect to the light polarized along the second birefringent axis 1282.
Thus, in accordance with this example, the light that originally propagated in the
horizontal polarization that was rotated to be in alignment with the first birefringent
axis 1280 incurs a relative phase shift of 90° with respect to any light that propagates
along the second birefringent axis 1282.
[0114] After passing through the quarter-wave plate 1274, the light passes through the second
Faraday rotator 1276 and is again rotated by 45° in the counterclockwise direction.
The light emerging from the second Faraday rotator 1276 passes through the second
collimating lens 1278 and is focused into the second end 1262 of the second PM optical
fiber 1258. It should be understood from the foregoing description that any light
output from the first PM optical fiber 1252 in the horizontal polarization enters
the second PM optical fiber 1258 in the vertical polarization. As discussed above,
the light entering the second PM optical fiber 1258 in the vertical polarization will
have propagated along the slow birefringent axis 1280 of the quarter-wave plate 1274
and will incur a relative π/2 phase difference with respect to light that propagates
along the fast birefringent axis 1282.
[0115] As indicated by its description, the non-reciprocal phase shifter 1250 operates in
a non-reciprocal manner because of the operation of the Faraday rotators 1272, 1276.
As described above, the light passing through the Faraday rotators 1272, 1276 from
the first PM fiber 1252 to the second PM fiber 1258 is rotated 45° counterclockwise
by each rotator with respect to the direction of propagation of the light shown in
Figure 25. If the Faraday rotators were reciprocal, light propagating through the
Faraday rotators 1272, 1276 in the opposite direction would also be rotated in the
counterclockwise direction with respect to the direction of propagation of the light;
however, because the Faraday rotators are non-reciprocal, the light is rotated in
the opposite direction (i.e., clockwise with respect to the propagation direction
of the light). The non-reciprocal effect is illustrated in Figure 27 for light passing
from the second end 1262 of the second PM fiber 1258, through the non-reciprocal phase
shifter 1250, to the second end 1256 of the first PM fiber 1252. Note that when viewed
as in Figure 27, the rotation appears to again be in the counterclockwise direction;
however, the light is now propagating toward the viewer. Thus, light emitted from
the second end 1262 of the second PM optical fiber 1258 in the vertical polarization
passes through the second collimating lens 1278 and through the second Faraday rotator
1276 and is rotated to an orientation in alignment with the second (fast) birefringent
axis 1282 of the quarter-wave plate 1274. Thus, the light originally in the vertical
polarization does not experience a relative delay as it propagates through the quarter-wave
plate 1274. After passing through the quarter-wave plate, 1274, the light passes through
the first Faraday rotator 1272 such that the light is rotated an additional 45° to
the horizontal polarization. The light is then focused through the first collimating
lens 1270 onto the second end 1256 of the first PM optical fiber 1252.
[0116] From the foregoing, it can be seen that the horizontally polarized light passing
in the first direction from the first PM fiber 1252 to the second PM fiber 1258 via
the non-reciprocal phase shifter 1250 propagates through the slow birefringent axis
1280 of the quarter-wave plate 1274 and experiences a relative phase delay of 90°
or π/2. The horizontally polarized light propagating in the first direction is rotated
such that that the light is oriented in the vertical polarization when it enters the
second PM fiber 1258. Conversely, vertically polarized light passing from the second
PM fiber 1258 to the first PM fiber 1252 via the non-reciprocal phase shifter 1250
in the second direction propagates through the fast birefringent axis 1282 of the
quarter-wave plate 1274 and does not experience a relative phase delay. The vertically
polarized light propagating in the second direction is rotated such that the light
is oriented in the horizontal polarization when it enters the first PM fiber 1252.
As will be discussed more fully below, the relative phase shift between the horizontally
polarized light propagating in the first direction with respect to the vertically
polarized light propagating in the second direction provides a π/2 phase bias.
[0117] Figures 28 and 29 illustrate an alternative embodiment of the non-reciprocal phase
shifter 1250, in which the first Faraday rotator 1272 is positioned between the quarter-wave
plate 1274 (now referred to as the first quarter-wave plate) and a second quarter-wave
plate 1294. In Figure 28, light from the second end 1256 of the first PM fiber 1252
is collimated by the first collimating lens 1270, as before. The light is originally
in the horizontal polarization. When the light passes through the first quarter-wave
plate 1274, it is converted to light having a circular polarization. The circular
polarized light passes through the first Faraday rotator 1272, which causes the circularly
polarized light to incur a phase shift of φ. In the preferred embodiment, the first
Faraday rotator 1272 is selected to cause a phase shift of π/4. The light from the
Faraday rotator 1272 remains circularly polarized and passes through the second quarter-wave
plate 1294, which converts the circularly polarized light to linearly polarized light
in the vertical polarization orientation. In addition to being in the vertical polarization,
the light has experienced a phase shift of φ (e.g., π/4).
[0118] Figure 29 illustrates the operation of the alternative embodiment of the non-reciprocal
phase shifter 1250 for light propagating in the opposite direction. In Figure 29,
vertically polarized light from the second end 1262 of the second PM fiber 1260 is
collimated by the second collimating lens 1278 and passes through the second quarter-wave
plate 1294. The second quarter-wave plate 1294 converts the vertically polarized light
to light having a circular polarization. The circularly polarized light passes through
the first Faraday rotator 1272 and experiences a phase shift as before. Because the
light is propagating through the first Faraday rotator 1272 in the opposite direction,
the light experiences an opposite phase shift of -φ (e.g., -π/4). The light from the
first Faraday rotator 1272 then passes through the first quarter-wave plate 1274,
where the circularly polarized light is converted to linearly polarized light with
a horizontal polarization. Thus, the light propagating in the two directions experiences
a total relative phase shift of 2φ (e.g., π/2), which has the same effect as the first
embodiment of the non-reciprocal phase shifter 1250 illustrated in Figures 26 and
27.
[0119] The effect of the non-reciprocal phase shifter 1250 on the orientation of the polarization
and the phase delay provides the biasing effect described above and explained again
in connection with Figure 24. As shown in Figure 24, the light entering the second
PM fiber 1258 in the vertical polarization is combined at the second PM coupler 1266
with the light that propagated through the common rung 712 from the first PM coupler
1264 to the second PM coupler 1266. For reasons that will become apparent in the following
discussion, it is desirable that the light entering the second PM coupler 1266 from
the common rung 712 have the same polarization as the light entering the second PM
coupler from the second PM fiber 1258. Thus, in the preferred embodiment, either the
second PM fiber 1258 or the common rung 712 is rotated by 90° so that the light in
the vertical polarization in the second PM fiber 1258 is oriented in the same direction
as the light in the horizontal polarization of the common rung 712. This is readily
accomplished by rotating the second end 1262 of the second PM fiber 1258 proximate
to the second collimating lens 1278 so that the vertically polarized light enters
the second end 1262 with its state of polarization oriented along the horizontal polarization
axis of the second PM fiber 1258. Thus, the light that exits the non-reciprocal phase
shifter 1250 in the vertical state of polarization is applied to the coupler 1266
as light in the horizontal state of polarization with respect to the polarization
axes of the coupler 1266. Accordingly, the light from the non-reciprocal phase shifter
1250 has the same state of polarization as the light from the common rung 712.
[0120] The light that passes through the common rung 712 and the light that passes through
the non-reciprocal phase shifter 1250 next enter the port 1124 of the polarization
beam splitter (PBS) 1104. The light in the horizontal polarization is output from
the port 1123 of the PBS 1104 to the fiber 740. The fiber 740 includes the delay loop
750 and is terminated at the Faraday rotating mirror (FRM) 1106. The delay loop 750
and the FRM 1106 operate as discussed above, and the reflected and delayed pulses
are returned to the port 1123 of the PBS 1104 in the vertical polarization. The pulses
are output from the port 1121 of the PBS 1104 to the array 716 via the fiber 720 and
propagate in the clockwise direction through the sensors 722(i) of the array 716.
[0121] The pulses are output from the array 716 via the fiber 714 and the depolarizer 1110
to the 2x2 coupler 1220 where the clockwise propagating light is combined with the
counterclockwise propagating light. The counterpropagating light also starts out as
horizontally polarized light. The light is depolarized and passes through the sensor
array 716. Light emerging from the sensor array 716 in the vertical polarization is
reflected by the PBS 1123 and is discarded via the port 1122 and the terminator 732.
Light emerging from the sensor array 716 in the horizontal polarization passes through
the PBS 1123, is delayed by the loop 750, and is rotated to the vertical polarization
by the FRM 1106. The return light, which is in the vertical polarization, is reflected
by the PBS 1123 to the port 1124 and is thus directed to the second PM coupler 1266.
A portion of the light passes through the delay loop 1269 of the common rung 712 and
a portion of the light passes through the non-reciprocal phase shifter 1250. As discussed
above, light entering the non-reciprocal phase shifter 1250 in the vertical polarization
propagates through the fast birefringent axis 1282 of the quarter-wave plate 1274
(Figure 27) and does not experience a relative phase delay. Thus, the two pulses of
counterclockwise light propagate to the coupler 1220 where they are combined with
the clockwise propagating light pulses. The light signals that passed through the
common rung 712 and the delay loop 1269 in both directions experience no relative
phase shift and combine as discussed above. The light signals that passed through
the non-reciprocal phase shifter 1250 in both directions experience a relative phase
shift of π/2 between the clockwise propagating signal and the counterclockwise propagating
signal and thus have a π/2 phase bias, as discussed above. At both outputs of the
coupler 1220, a portion of the two pulses of light returning from the sensor array
1200 is directed to the polarizer 1224, and the remaining portion is directed to the
polarizer 1232. The role of the two polarizers 1224 and 1232 is to ensure that the
light entering the loop has the same polarization as the light leaving the loop, which
guarantees reciprocity. As described earlier, the two pulses reaching the detector
1230 are in phase quadrature, which allows the use of a number of signal processing
techniques well-known in the art to avoid signal fading. Similar comments apply to
the detector 1226. In the embodiment of Figure 24, the generation of two pulses in
phase quadrature is the main reason for incorporating the rung containing the non-reciprocal
phase shifter 1250.
[0122] Figures 30-36 illustrate further alternative embodiments of the present invention
in which a folded Sagnac sensor array utilizes polarization-based biasing for multiple
detectors, wherein each detector has a bias point which can be set independently of
the bias points of the other detectors. The embodiments of Figures 30-36 include the
sensor array 716, which was described in detail above. It should be understood that
other configurations of amplified sensor arrays can also be used in place of the sensor
array 716 in the embodiments of Figures 30-36.
[0123] In a folded Sagnac sensor array 1300 illustrated in Figure 30, a polarized fiber
superfluorescent source (SFS) 1310 is coupled to a polarization controller 1312 via
a fiber 1314. The fiber 1314 further couples the polarization controller 1312 to a
first port of a 2x2 coupler 1316. A second port of the coupler 1316 is an output port,
which will be discussed below. A third port of the coupler 1316 is coupled via a fiber
1318 to a non-reflective terminator 1320. A fourth port of the coupler 1316 is coupled
to a first port 1330 of a polarization beam splitter (PBS) 1332 via a common array
input/output fiber 1334. A second port 1336 of the polarization beam splitter 1332
is coupled to a first horizontal polarizer 1338. The first horizontal polarizer 1338
is coupled to the second array input/output fiber 720 of the array 716. A third port
1340 of the polarization beam splitter 1332 is connected to a common delay fiber 1342,
which is formed into a delay loop 1344 and which is terminated at a Faraday rotating
mirror (FRM) 1346. A fourth port 1348 of the polarization beam splitter 1332 is coupled
to a second horizontal polarizer 1350 and then to a depolarizer 1352. The depolarizer
1352 is coupled to the first array input/output fiber 714.
[0124] The second port of the coupler 1316 is coupled to a detector subsystem 1360 via a
fiber 1362. In the embodiment of Figure 30, the detector subsystem 1360 comprises
a 1×n coupler 1364 which has a single input port that receives the light from the
second port of the coupler 1316. A first output port of the 1xn coupler 1364 is coupled
to a polarization controller 1366. The polarization controller 1366 is coupled to
a polarizer 1368, which is in turn coupled to a first detector 1370. A second output
port of the 1xn coupler 1364 is coupled to a polarization controller 1372. The polarization
controller 1372 is coupled to a polarizer 1374, which is coupled to a second detector
1376. Additional polarization controllers, polarizers and detectors (not shown) can
be connected to additional ports (not shown) of the 1 xn coupler 1364.
[0125] The folded Sagnac sensor array 1300 of Figure 30 operates in the following manner.
The polarized SFS 1310 provides a polarized output signal which passes through the
polarization controller 1312 via the fiber 1314. The polarization controller 1312
is adjustable to vary the polarization to a desired state of polarization. For example,
in Figure 30, the state of polarization is adjusted to provide linearly polarized
light oriented at 45° with respect to the vertical and horizontal axes at the input
to the polarization beam splitter 1332. The light remains in the fiber 1314 and is
provided as the input to the coupler 1316. The coupler 1316 couples approximately
50 percent of the incoming light to the first output fiber 1318 and is thus discarded
at the non-reflective terminator 1320. The coupler 1316 couples approximately 50 percent
of the incoming light to the common array input/output fiber 1334.
[0126] The common array input/output fiber 1334 guides the light to the polarization beam
splitter 1330, which reflects horizontally polarized light to the second port 1336
and which passes vertically polarized light to the third port 1340. The reflected
horizontally polarized light from the second port 1336 passes through the first horizontal
polarizer 1338 to the second array input/output fiber 720 and propagates in a clockwise
direction through the array 716. The clockwise propagating light exits the array 716
via the depolarizer 1352 and the array input/output fiber 714. As discussed above,
the depolarizer 1352 assures that the exiting light is substantially equally distributed
in the horizontal polarization mode and the vertical polarization mode after passing
through the sensors in the array 716. The clockwise propagating light then passes
through the second horizontal polarizer 1350, which eliminates the portion of the
light in the vertical polarization. The clockwise propagating light in the horizontal
polarization then enters the fourth port 1348 of the polarization beam splitter 1330
and is reflected to the third port 1340 to propagate in the common delay fiber 1342.
The returning clockwise light passes through the delay loop 1344 to the Faraday rotating
mirror 1346 where it is reflected as vertically polarized light. The vertically polarized
light returns to the third port 1340 of the polarization beam splitter 1332 and is
passed through to the first port 1330.
[0127] As discussed above, the light which was originally incident at the first port 1330
of the polarization beam splitter 1332 was oriented at approximately 45° to the horizontal
and vertical polarizations. Thus, approximately 50 percent of the light corresponding
to the vertically polarized component of the light passed through the polarization
beam splitter 1332 to the third port 1340 and thus to the common delay fiber 1342.
The vertically polarized light propagates through the delay loop 1344 and is reflected
by the Faraday rotating mirror 1346 as horizontally polarized light. The reflected
horizontally polarized light passes through the delay loop 1344 and back to the third
port 1340 of the polarization beam splitter 1332. Because the light is horizontally
polarized, the light is reflected to the fourth port 1348 of the polarization beam
splitter 1332 and is thus caused to propagate via the first array input/output fiber
714, through the second horizontal polarizer 1350, through the depolarizer 1352 and
into the array 716 to propagate therein in a counterclockwise direction. The depolarizer
1352 assures that the counterclockwise propagating light has components in all polarizations
so that when the counterclockwise propagating light emerges from the array 716, there
will be at least a portion of the light in the horizontal polarization.
[0128] The counterclockwise propagating light emerges from the array 716 via the second
array input/output fiber 720, and the horizontally polarized component of the light
passes through the first horizontal polarizer 1338, which eliminates the light at
other polarization orientations. The horizontally polarized light resulting from the
counterclockwise propagating portion of the light enters the second port 1336 of the
polarization beam splitter 1332 and is reflected to the first port 1330 of the polarization
beam splitter 1332 where it is combined with the vertically polarized light which
resulted from the clockwise propagating portion of the light.
[0129] The combined light propagates to the fourth port of the coupler 1316 where approximately
50 percent of the combined light is coupled to the second port of the coupler 1316
and thus to the detector subsystem 1360 via the fiber 1362. The 1xn coupler 1364 divides
the light into N portions. For example, in Figure 30, N is equal to 2, and a first
portion of the light is coupled to the polarization controller 1366 to propagate through
the polarizer 1368 to the first detector 1370, and a second portion of the light is
coupled to the polarization controller 1372 to propagate through the polarizer 1374
to the second detector 1376. The orientations of the polarization controllers 1366,
1372 and the polarizers 1368, 1374 can be adjusted to bias the optical signals incident
on the first detector 1370 and the second detector 1376 at different phases. For example,
the signal applied to the second detector 1376 can be biased to be in quadrature with
the signal applied to the first detector 1370 so that when one signal has minimum
sensitivity, the other signal has maximum sensitivity, and vice versa.
[0130] As discussed above, each of the two signal portions travels the same distance through
the array 716, through the common delay fiber 1342, and through the delay loop 1344.
Thus, in the absence of perturbations caused by acoustic signals or other noise impinging
on the sensors in the array 716, the two portions will be in phase and will constructively
interfere to generate a combined optical signal having a linear polarization of 45°;
however, the light has a state of polarization orthogonal to the original state of
polarization. Thus, if the original state of polarization was +45°, then the state
of polarization of the output light (again in the absence of a phase perturbation)
is -45°.
[0131] In the presence of an acoustic signal, the clockwise propagating light and the counterclockwise
propagating light experience a relative phase shift. With increasing relative phase
shift, the state of polarization of the two interfering beams changes from -45° linear
polarization to left-hand circular polarization to +45° polarization to right-hand
circular polarization and back to -45° polarization. The progression through these
four states of polarization define a circle on the Poincaré sphere. The state of polarization
at the output of the polarization beam splitter 1332 corresponds to a point along
this circle on the Poincaré sphere whose location on the circle is a function of the
acoustically-induced non-reciprocal phase shift.
[0132] After traveling from the output of the polarization beam splitter 1332, through the
common array input/output fiber 1334, through the coupler 1316 and to the detector
subsystem 1360, the state of polarization of the combined signal is altered arbitrarily
by the unknown birefringence of the fiber 1334. The polarization controller 1366 proximate
to polarizer 1368 in front of the first detector 1370 and the polarization controller
1372 proximate to the polarizer 1374 in front of the second detector 1376 are used
to re-orient the states of polarization to a respective selected state of polarization
for each detector 1370, 1376. The polarization controllers 1366, 1372 are set, for
example, when no acoustic signals are applied to the array 716, and thus no relative
phase shift is introduced to the counterpropagating optical signals.
[0133] For example, to provide a bias point of ±90° for the first detector 1370, the polarization
controller 1376 is set so that when the combined light at the output of the polarization
beam splitter 1332 has a left-hand circular state of polarization, the first detector
1370 detects either a maximum intensity or a minimum intensity of the light. For other
states of polarization of the output light, the first detector 1370 detects light
having an intensity between the maximum intensity and the minimum intensity.
[0134] As a further example, the second detector 1376 can advantageously be set to a different
bias point, such as, for example, 0° and 180°. For this bias point, the polarization
controller 1372 is set so that when the light at the output of the polarization beam
splitter 1332 has a -45° state of polarization, the second detector 1376 detects either
a maximum intensity or a minimum intensity of the light. For other states of polarization
of the output light, the second detector 1376 detects light having an intensity between
the maximum intensity and the minimum intensity.
[0135] It should be understood that the light applied to the input of the polarization beam
splitter 1332 can have a state of polarization other than ±45°. For example, if the
input light has an original left-hand circular state of polarization, the polarization
controllers 1366, 1372 are set accordingly to provide the appropriate bias points
to the first detector 1370 and the second detector 1376.
[0136] Figure 31 illustrates an alternative configuration of a folded Sagnac sensor array
1300', which is substantially similar to the folded Sagnac sensor array 1300 of Figure
30. In the folded Sagnac sensor array 1300' of Figure 31, the depolarizer 1352 is
located in the second array input/output fiber 720 rather than in the first array
input/output fiber 714. Because of the reciprocal structure of the sensor array 716,
the relocation of the depolarizer 1352 to the fiber 720 does not change the overall
operation of the folded Sagnac sensor array 1300' with respect to the operation of
the folded Sagnac sensor array 1300. The operation of the folded Sagnac sensor array
1300' is similar to the operation of the folded Sagnac sensor array 1300 and will
not be described in detail herein.
[0137] Figure 32 illustrates a further alternative embodiment of a folded Sagnac acoustic
sensor array 1400, which is similar to the folded Sagnac sensor array 1300 of Figure
30, and like elements have been numbered accordingly. Unlike the folded Sagnac sensor
array 1300, the folded Sagnac sensor array 1400 replaces the 2x2 coupler 1316 with
a polarization independent optical circulator 1410. The optical circulator performs
a similar function as the 2x2 coupler 1316; however, in the folded Sagnac sensor array
1300 approximately 50 percent of the input light is lost when the input light is split
at the coupler 1316 and approximately 50 percent of the output light is lost when
it is split at the coupler 1316. In the embodiment 1400, substantially all the input
light is passed from the polarized SFS 1310 through the circulator 1410 to the polarization
beam splitter 1332 and substantially all the output light is passed from the polarization
beam splitter 1332 through the circulator 1410 to the detector subsystem 1360.
[0138] Figure 33 illustrates an alternative configuration of a folded Sagnac sensor array
1400', which is substantially similar to the folded Sagnac sensor array 1400 of Figure
32. In the folded Sagnac sensor array 1400' of Figure 33, the depolarizer 1352 is
located in the second array input/output fiber 720 rather than in the first array
input/output fiber 714. Because of the reciprocal structure of the sensor array 716,
the relocation of the depolarizer 1352 to the fiber 720 does not change the overall
operation of the embodiment 1400' with respect to the operation of the folded Sagnac
sensor array 1400. Thus, the operation of the folded Sagnac sensor array 1400' will
not be described in detail herein.
[0139] Figure 34 illustrates a further alternative embodiment of a folded Sagnac sensor
array 1600 in accordance with the present invention, which includes a combined input/output
subsystem 1610 which is coupled to the array 716 in a manner similar to the manner
described above in connection with Figures 30-33.
[0140] In Figure 34, a polarized source 1620 provides linearly polarized input light along
an axis of a polarization maintaining fiber 1622. The polarization maintaining fiber
1622 is rotated such that the polarization axis is oriented at ±45° with respect to
the vertical polarization axis of the input output system 1610. The light from the
fiber 1622 is coupled to the input/output subsystem 1610 via a first collimating lens
1630. The first collimating lens 1630 directs the light toward a first port 1634 of
a first polarization beam splitter (PBS) 1632, which also has a second port 1636,
a third port 1638 and a fourth port 1640. The second port 1636 directs a portion of
the input light toward a first 45° Faraday rotator (45° FR) 1642. The third port 1638
directs a portion of the input light toward a second 45° Faraday rotator 1644. As
will be described below, the fourth port 1640 directs a selected portion of output
light to a detection subsystem 1650.
[0141] The light passing through the first Faraday rotator 1642 is collimated by a second
collimating lens 1660 and is coupled into the array input/output fiber 720 and thus
propagates to the sensor portion of the array 716 to propagate in a clockwise direction
therein.
[0142] The light passing through the second Faraday rotator 1644 passes through a half-wave
(λ/2) plate 1662. The half-wave plate 1662 has first and second birefringent axes
(not shown). One of the birefringent axes is oriented at an angle of 22.5° with respect
to the vertical polarization axis of the incoming light and at -22.5° with respect
to the 45° polarization of the light traveling toward it from the source (i.e., the
axis lies between vertical and the polarization of the light). The purpose of this
orientation will be described below. The light passing through the half-wave plate
1662 enters a first port 1672 of a second polarization beam splitter 1670, which also
has a second port 1674, a third port 1676 and a fourth port 1678. As discussed below,
the second port 1674 is not coupled to additional elements. Light output from the
third port 1676 is directed toward a third collimating lens 1680. Light output from
the from the fourth port 1678 is directed toward a fourth collimating lens 1682.
[0143] The light passing through the fourth collimating lens 1682 is coupled into the first
array input/output fiber 714 and passes through the depolarizer 1352 into the sensor
portion of the array 716 to propagate in a counterclockwise direction therein.
[0144] The light passing through the third collimating lens 1680 is focused onto the end
of the common delay fiber 1342; propagates through the delay loop 1344 to the Faraday
rotating mirror 1346, back through the delay loop 1344 and back to the collimating
lens 1680. The reflected light is thus directed back into the third port 1676 of the
second polarization beam splitter 1670.
[0145] As discussed above, the light from the fourth port 1640 of the first polarization
beam splitter 1632 enters the detection subsystem 1650. The detection subsystem 1650
comprises a first beam splitter 1690, a second beam splitter 1692, a first birefringent
element 1694, a second birefringent element 1696, a first detector 1698, a second
detector 1700, a first polarizer 1702, and a second polarizer 1704. A first percentage
of the light from the fourth port 1640 is reflected by the first beam splitter 1690
and passes through the first birefringent element 1694 and the first polarizer 1702
to the first detector 1698. The remaining portion of the light from the fourth port
1640 passes through the first beam splitter 1690 and is incident on the second beam
splitter 1692 where a second percentage of the light is reflected by the second beam
splitter 1692 to pass through the second birefringent element 1696 and the second
polarizer 1704 to the second detector 1700. The remaining portion of the light passes
through the second beam splitter 1692 to additional elements (not shown). If only
two detectors are provided, the first percentage of coupling is advantageously 50
percent and the second percentage is advantageously 100 percent so that both detectors
1698,1700 receive approximately the same amount of light. If a third detector (not
shown) is included, then the first percentage is advantageously about 33□ percent,
and the second percentage is advantageously about 50 percent so that the second detector
1700 detector also receives approximately 33□ percent of the original light. The third
detector would then receive the remaining 33□ percent.
[0146] The folded Sagnac sensor array 1600 of Figure 34 operates in the following manner.
As discussed above, the light incident on the first lens 1630 is oriented at 45° to
the vertical and horizontal polarization axes. Thus, the light passing through the
lens 1630 and entering the first port 1634 of the first polarization beam splitter
1632 has a component in the horizontal state of polarization and a component in the
vertical state of polarization. The horizontal component is reflected by the polarization
beam splitter 1632 to the second port 1636, and the vertical component is passed through
the polarization beam splitter 1632 to the third port 1638.
[0147] The horizontal component from the second port 1636 passes through the first Faraday
rotator 1642, and the state of polarization is rotated by 45° in a first direction
(e.g., clockwise) so that the light emerging from the first Faraday rotator 1642 and
incident on the second lens 1660 has a linear state of polarization at 45°. The light
passes through the second lens 1660 and enters the second array input/output fiber
720 to propagate in the clockwise direction through the array 716. The light may encounter
changes in polarization within the array 716. Thus, as described above, the light
exiting the array 716 via the first array input/output fiber 714 passes through the
depolarizer 1352, which assures that at least a portion of the light is in the horizontal
and vertical states of polarization.
[0148] The clockwise propagating light from the first array input/output fiber 714 enters
the input/output subsystem 1610 via the fourth lens 1682 and is incident on the second
polarization beam splitter 1670. The vertical component of the light passes through
the second polarization beam splitter 1670 is output from the second port 1674 and
is discarded. The horizontally polarized component of the light is reflected to the
third port 1676 of the second polarization beam splitter 1670 and passes through the
third lens 1680 to the common delay fiber 1342 to cause the light to propagate through
the delay loop 1344, be reflected by the Faraday rotating mirror 1346 in the vertical
state of polarization, pass back through the delay loop 1344 and the common delay
fiber 1342 to the third lens 1680. The reflected light in the vertical state of polarization
passes from the third port 1676 to the first port 1672 of the second polarization
beam splitter 1670, passes through the half-wave plate 1662 to the second Faraday
rotator 1644 to the third port 1638 of the first polarization beam splitter 1632.
Because the half-wave plate 1662 is oriented with one of its birefringent axes at
22.5° with respect to the vertical polarization axis, the vertical light incident
on the half-wave plate 1662 is caused to be mirrored about the birefringent axis so
that the state of polarization of the light emerging from the half-wave plate 1662
is oriented at 45° with respect to vertical and horizontal axes. The second Faraday
rotator 1644 rotates the state of polarization by a further 45° to cause the light
emerging from the second Faraday rotator 1644 and incident on the third port 1638
of the first polarization beam splitter 1632 to have a horizontal state of polarization.
Thus, the light entering the third port 1638 is reflected to the fourth port 1640
and enters the detection subsystem 1650 in the horizontal state of polarization.
[0149] As set forth above, the vertical component of the input light incident on the first
port 1634 of the first polarization beam splitter 1632 passes through to the third
port 1638. The state of polarization of the light is rotated by 45° by the second
Faraday rotator 1644 to a 45° state of polarization with respect to the vertical and
horizontal polarization axes. The state of polarization of the light is then mirrored
about the birefringent axis of the half-wave plate 1662 so that the state of polarization
of the light emerging from the half-wave plate is again oriented in the vertical direction.
It will be understood by one skilled in the art that the non-reciprocal action of
the second Faraday rotator 1644 causes the vertically polarized light that passes
from left to right through the second Faraday rotator 1644 and then through the half-wave
plate 1646 to first be rotated to a 45° state of polarization and then to be mirrored
back to a vertical state of polarization. In contrast, the vertically polarized light
that passes from right to left is first mirrored by the half-wave plate 1646 to a
45° state of polarization and is then rotated by the second Faraday rotator 1644 to
a horizontal state of polarization.
[0150] The vertically polarized light from the half-wave plate 1662 enters the first port
1672 of the second polarization beam splitter 1670 and passes through to the third
port 1676 to the third lens 1680. The vertically polarized light passes through the
common delay fiber 1342, through the delay loop 1344, to the Faraday rotating mirror
1346, and is reflected back through the delay loop 1344 and the common delay fiber
1342 as horizontally polarized light. The horizontally polarized light passes through
the third lens 1680 to the third port 1676 of the polarization beam splitter 1670.
The horizontally polarized light is reflected to the fourth port 1678 and passes through
the fourth lens to 1682 to the first array input/output fiber 714 and through the
depolarizer 1352 to propagate in a counterclockwise direction through the array 716.
[0151] The counterclockwise propagating light emerges from the array 716 via the second
array input/output fiber 720 and passes through the second lens 1660 to the first
Faraday rotator 1642. The first Faraday rotator 1642 rotates state of polarization
of the light by 45°. Since the light was effectively depolarized by the depolarizer
1352, the tight that passes through the first Faraday rotator 1642 to the second port
1634 of the first polarization beam splitter 1632 includes light that has horizontally
and vertically polarized components. The horizontally polarized components of the
light are reflected to the first port 1634 and are output through the first lens 1630
to the input fiber 1622. An isolator (not shown) is advantageously included to absorb
the light.
[0152] The vertically polarized components of the counterclockwise propagating light entering
the second port 1636 of the first polarization beam splitter 1632 pass to the fourth
port 1640 and are combined with the horizontally polarized components of the clockwise
propagating light. As discussed above in connection with Figure 30, if the counterpropagating
light experiences no relative phase shift, the light is combined as linearly polarized
light at a 45° state of polarization. A relative phase shift causes the state of polarization
to vary, as further discussed above.
[0153] The birefringent elements 1694, 1696 are included to selectively bias the light incident
on the detectors 1698, 1900 by introducing a relative phase shift for the light in
the two different polarizations (e.g., the horizontal and vertical polarizations,
the +45° and -45° polarizations, or the left-hand circular and right-hand circular
polarizations). The birefringent elements may advantageously comprise linear or circular
waveplates (e.g., quarter-wave plates, half-wave plates, Faraday rotators, or the
like).
[0154] Figure 35 illustrates an embodiment of a folded Sagnac acoustic sensor array 1750
similar to the folded Sagnac acoustic sensor array 1600 of Figure 34, and like elements
are identified with the same numbers as in Figure 34. Unlike the embodiment of Figure
34, the folded Sagnac acoustic sensor array 1750 includes an unpolarized light source
1720 instead of the polarized light source 1620. In order to utilize the unpolarized
light source 1720, the folded Sagnac acoustic sensor array 1750 includes a 45° polarizer
1730 between the first collimating lens 1630 and the first polarization beam splitter
1632. The 45° polarizer 1730 causes the light incident on the first port 1634 of the
first polarization beam splitter 1632 to be oriented at 45° and to thus have substantially
equal components in the horizontal and vertical polarizations. Thus, the folded Sagnac
acoustic sensor array 1750 of Figure 35 operates in substantially the same manner
as the folded Sagnac acoustic sensor array 1600 of Figure 34, and the operation of
the folded Sagnac acoustic sensor array 1750 will not be described in further detail.
[0155] Figure 36 illustrates a further embodiment of a folded Sagnac acoustic sensor array
1800 similar to the folded Sagnac acoustic sensor arrays 1600 and 1750 of Figures
34 and 35, respectively, and like elements are identified with the same numbers as
in Figures 34 and 35. Unlike the embodiments of Figures 34 and 35, the folded Sagnac
acoustic sensor array 1800, the light signals passing through the polarizers 1702
and 1704 are not directed to the detectors 1698 and 1700. Rather, the folded Sagnac
acoustic sensor array 1800 includes a collimating lens 1810 positioned proximate to
the polarizer 1702 and a collimating lens 1812 positioned proximate to the polarizer
1704. The collimating lens 1810 directs the light from the polarizer 1702 into a first
end 1822 of a fiber 1820. The fiber 1820 has a second end 1824 proximate to the first
detector 1698 such that the light entering the fiber 1820 from the collimating lens
1810 is incident on the first detector 1698. Similarly, the collimating lens 1812
directs the light from the polarizer 1702 into a first end 1832 of a fiber 1830. The
fiber 1830 has a second end 1834 proximate to the second detector 1700 such that the
light entering the fiber 1830 from the collimating lens 1812 is incident on the second
detector 1700. By including the collimating lenses 1810 and 1812 and the fibers 1820
and 1830, the fibers are able to transport the light for a distance to the detectors
1698 and 1700 so that the detectors may be located in remote locations proximate to
the detection electronics (not shown).
[0156] Note that in Figures 34, 35 and 36, the depolarizer 1352 can be relocated from the
first array input/output fiber 714 to the second array input/output fiber 720 without
significantly affecting the operating characteristics of the folded Sagnac acoustic
sensor array 1600, the folded Sagnac acoustic sensor array 1750 or the folded Sagnac
sensor array 1800.
[0157] Note further that the foregoing embodiments have been described in connection with
superfluorescent light sources. One skilled in the art will appreciate that other
light sources (e.g., laser sources) may also be advantageously used.
[0158] Although the foregoing description of the array in accordance with the present invention
has addressed underwater acoustic sensing, it should be understood that the present
invention can be used to sense any measurand which can be made to produce non-reciprocal
phase modulations in a fiber. If, for example, the hydrophones were replaced with
an alternative sensing device which responds to a different measurand, the array would
detect that measurand in the same manner as acoustic waves are detected. The array
of the present invention can be advantageously used to sense vibrations, intrusions,
impacts, chemicals, temperature, liquid levels and strain. The array of the present
invention may also be used to combine a number of different sensors located at either
the same place or located in different places (e.g., for the detection of various
faults at various points along the hull of a ship or a building). Other exemplary
applications include the detection and tracking of moving automobiles on highways
or airplanes on airstrips for traffic monitoring and control.
[0159] Although described above in connection with particular embodiments of the present
invention, it should be understood the descriptions of the embodiments are illustrative
of the invention and are not intended to be limiting. Various modifications and applications
may occur to those skilled in the art without departing from the scope of the invention
as defined in the appended claims.